If ARC Raiders suddenly throws up messages like An illegal mutation was rejected by the backend or “Illegal Backend Mutation Error,” it looks and sounds a lot scarier than it is. Players see pop‑ups looping over the UI, failed actions in the stash, and inventory rollbacks that make it feel like progress is evaporating in real time.
Under the hood, this family of errors is all about a mismatch between what your game client thinks your account state is and what the ARC Raiders servers are willing to accept.
What ARC Raiders means by an “illegal mutation”
ARC Raiders is built as an online extraction shooter with almost everything tied to the backend: your inventory, stash, crafting, repairs, quest progress, and loadouts all live on Embark’s servers, not in a local save file. Every time you sell an item, craft a shield recharger, repair a gun, or change a loadout, the client sends a “mutation” request: “here’s how I want my account state to change.”
When the server replies with an error such as:
An illegal mutation was rejected by the backend. The inventory has been rolled back to its previous state. We have to redirect you back to the Main Hub.it is doing three things at once:
| What you see | What the backend is doing | Practical impact |
|---|---|---|
| “Illegal mutation” / ARBG0000 pop‑up | Rejecting a state change it considers invalid or out of sync | Action fails, pop‑up may spam and block UI |
| Message about inventory rollback | Reverting your account to last known good snapshot | Recent trades, crafts, or drops can disappear |
| Kick back to Main Hub | Forcing a clean resync of your session | Must re‑enter menus or matchmaking |
Most of the time, this isn’t a ban warning or an anti‑cheat verdict. Players hit it while doing mundane things: selling an air freshener, combining ammo stacks, crafting shield rechargers, or repairing a weapon. The error name sounds accusatory; the reality is usually a boring desync.
How the illegal mutation errors show up in play
Players report a few consistent patterns:
| Context | Typical symptom | What happens next |
|---|---|---|
| After a patch or hotfix | “Illegal Backend Mutation Error” when logging in or opening stash | Inventory appears rolled back to an earlier state |
| While managing inventory | Error when selling items, merging stacks, or recycling (e.g., rubber pads) | Action fails, or appears to work then gets undone |
| During crafting or repairs | Craft shows as successful but items don’t appear; later the error triggers | Backend restores one of the intermediate states, sometimes duplicating or losing items |
| Editing a loadout | Pop‑up loops every time a change is saved | Loadout silently resets, making it impossible to lock in changes |
| Immediately after completing tutorial or quests | Error tied to quest completion or objective turn‑in | Progress feels “stuck,” with repeated prompts or pop‑ups |
In the worst cases, players get the ARBG0000 / illegal backend mutation pop‑up so frequently that they can’t click any other UI element. The game is technically “running,” but the menus are unusable, and you’re effectively locked out of play.
Why inventory rollbacks keep happening
When the client and server disagree about your account state, ARC Raiders resolves the conflict by trusting the backend. That’s why some players see actions “work” locally—items leave the stash, currency changes, a repaired gun looks fine—then get slammed by an error and find the game reverted several minutes of changes.
Desync can happen for a few reasons:
- Network instability or momentary disconnects while mutating inventory
- High server load, especially during widespread outages and spikes in player count
- Bugs in how specific actions (like stacking ammo or recycling certain items) are validated server‑side
- Version drift right after an update, where the client talks slightly differently to the backend than expected
During larger outages, thousands of players report a mix of “Online connection error” prompts and illegal mutation messages right before being redirected to the Main Hub. Those events are less about your particular account and more about the game’s backend being under stress.
Is this an anti‑cheat or ban risk?
The wording of “illegal mutation” leads to quick accusations about save editing or cheating. In practice, players are seeing these errors even when they are playing normally and never touching local files.
What the error reliably indicates is:
- The server received a change to your inventory, stash, or account state that it refused to apply.
- The backend decided your safest version is an earlier snapshot, so it rolled you back.
- The client is out of sync and needs a forced refresh, sometimes via a kick to the Main Hub.
That does not automatically mean a ban is attached. It is a safeguard for data integrity first. If an account were flagged for cheating or tampering, that would be handled by separate systems and messaging. The players who get these errors most often are those unlucky enough to sit at the intersection of server issues and inventory‑heavy activities, not just those trying to exploit the game.
Known workarounds players are using
There is no guaranteed, universal fix yet, but a few patterns have helped some players get unstuck long enough to continue playing.
| Workaround | How it works | When it helps | Trade‑offs |
|---|---|---|---|
| Force a sync failure by toggling internet | Launch the game, then briefly disconnect your internet before the usual crash/error moment. Let the sync fail, then reconnect and resume. | ARBG0000 or illegal mutation errors that loop on login or in hub UI | Relies on timing; may need a few tries, and won’t fix underlying bugs. |
| Click through pop‑ups with a macro | Use an auto‑clicker to rapidly dismiss repeated pop‑ups, slipping clicks onto UI elements between each prompt. | When menus are spammed by the error but you can still technically interact between flashes | Fiddly and not officially supported; can be unreliable and is not a permanent fix. |
| Complete the affected quest or objective | Persist through the error spam long enough to finish the mission or quest tied to your bugged state. | Errors that started right after a particular quest step or tutorial completion | Requires enduring instability; not always clear which quest is the culprit. |
| Clean up “dirty” inventory stacks | Merge partial stacks of items like ammo so that you hold one incomplete stack instead of many non‑max stacks. | Errors that appear while adjusting stash or loadout, especially when many micro‑changes are pending | More of a mitigation: may reduce how often the issue triggers rather than eliminating it. |
| Create a fresh account | Set up a new Steam and Embark account and start over. | Severe, persistent errors tied to one account even after patches | All progress on the original account is effectively lost until the backend is fixed. |
Steps to try if you are currently stuck
When the game becomes unplayable due to repeated “illegal mutation” or ARBG0000 errors, the realistic options are limited, but there is still a sensible order of operations:
1. Rule out local quick fixes (even if they’re unlikely)
Basic client‑side actions rarely help with this class of error, but they are low risk:
- Fully close the game and relaunch.
- Power‑cycle your router and reconnect.
- On PC, verify game files through your platform launcher.
Players report that reinstalls almost never fix illegal backend mutation issues by themselves, because the problem sits on the account state, not on local files.
2. Try forcing a fresh sync
If the error hits at a predictable moment—for example, shortly after loading into the hub—some players have had success by deliberately breaking then restoring connectivity:
- Launch ARC Raiders and log in as usual.
- Right before the moment you usually see the error, briefly disconnect your PC or console from the internet (unplug Ethernet, disable Wi‑Fi, or use your router’s on/off switch).
- Wait for the game to complain about lost connection.
- Reconnect to the internet and let the game attempt to resync.
The goal is to force the backend to treat your next session as a clean read of whatever it considers the canonical state, instead of trying to apply a half‑broken sequence of queued mutations.
3. Reduce noisy inventory changes
Once you are back in the hub or menus, it helps to simplify what you ask the server to do in one go:
- Open your stash and check for multiple partial stacks of the same item, especially ammo.
- Manually combine stacks until you have as many full stacks as possible and a single partial stack.
- Avoid rapid‑fire selling, crafting, or recycling; make one change at a time and give the game a few seconds to settle.
This doesn’t eliminate desync risks, but it reduces the number of tiny mutations the backend has to evaluate and apply, which has helped some players lower how often errors appear.
4. Push through and complete the bugged step
When the error started immediately after a tutorial, quest step, or trader run, some players managed to stabilize their account by:
- Using rapid clicking (or a macro) to keep dismissing pop‑ups.
- Finishing the specific quest or trader cycle that always seems to precede the error.
- Returning to the hub and then tidying inventory as described above.
Once the problematic step is fully resolved and the backend has a clean snapshot that includes its completion, the error sometimes disappears or becomes much less frequent.
5. Consider a temporary new account only as a last resort
Where illegal mutation errors are tied tightly to a particular account and persist across patches, one of the few ways players have continued to enjoy ARC Raiders has been to create a fresh account (both on their store platform and in Embark’s ecosystem) and start over.
This is a heavy option with serious trade‑offs:
- You lose access to all progress, unlocks, and gear on the original account until Embark fixes the backend state.
- You are effectively treating the bugged account as “quarantined” and hoping a future backend cleanup restores it.
It is only worth considering if the original account is totally unplayable and none of the lighter workarounds have helped.
Where outages and server load fit into this
Illegal mutation messages are not only an individual problem. They also show up during large‑scale outages when ARC Raiders’ servers are under heavy load. During those spikes, tens of thousands of players at once report:
- “Online connection error” prompts when logging in or matchmaking
- Illegal mutation messages followed by forced returns to the Main Hub
- Inventory rollbacks hitting right after extracting with valuable blueprints or loot
In those windows, there is nothing useful to tweak locally. The only realistic options are to stop making important inventory changes, avoid high‑stakes extraction runs, and wait for Embark to stabilize the backend.
What to expect until Embark patches the backend
Illegal mutation and ARBG0000 errors are symptoms of how strictly ARC Raiders’ backend guards your account data. That strictness protects against genuine tampering and some classes of bugs, but it also means innocent players get caught in the net when desyncs or validation issues slip through.
Until Embark ships targeted server‑side fixes and account‑state repairs, the pattern is likely to continue:
- Most players will see occasional illegal mutation messages tied to lag or timeout events, often with no lasting damage.
- A smaller group will hit repeatable account‑level issues that can soft‑lock menus or repeatedly reset loadouts.
- During outages or surges in player count, the error will spike more broadly as the backend refuses changes it can’t safely commit.
For now, the safest approach is to treat any appearance of “illegal mutation” or “Illegal Backend Mutation Error” as a warning that the server did not trust a recent change. Slow down actions that touch your inventory, avoid making major stash reorganizations during obvious server instability, and use the sync‑forcing and cleanup tricks to regain control when the UI locks up.
