Rainbow Six Siege Launch Crash on Windows 11: Safely Remove KB5124010 and KB5121794

That sounds like a quick "remove the update and carry on" job. It isn't quite. The two KB numbers are very different kinds of update, and removing the second one may do more than a typical rollback. Here's what each one is, how to check which you have, and how to apply the workaround safely.

What Ubisoft actually said​

• Symptom: Siege crashes on PC when you try to launch it.
• Cause (per Ubisoft): the latest Windows preview updates.
• Workaround: uninstall KB5124010 and KB5121794, then restart.
• Status: Ubisoft calls this a temporary workaround while it works on a fix.

Insider Gaming couldn't independently retrieve Ubisoft's original post, so the wording above comes from that outlet's report. Neither Ubisoft nor Microsoft has publicly given a code-level explanation. Nobody has officially confirmed whether the anti-cheat, a kernel driver, or something else is the actual point of failure.

Section summary: Ubisoft blames the latest Windows 11 preview updates for the launch crashes and suggests removing two of them for now. The technical root cause hasn't been disclosed.

Siege isn't alone: KB5124010 already has a record​

This isn't the first game reported to break on KB5124010. Last week, TweakTown, citing Neowin, reported that multiple user reports claim that Windows 11's latest preview update is causing Battlefield 6, WARDOGS, and some other games with anti-cheat support to crash. TweakTown added that there's still no full confirmation that anti-cheat issues are causing these games to crash, but it appears all of the games that are proving problematic with the Windows update do have some form of anti-cheat.

At least one other developer had already gone on record before Ubisoft. BottleneckPC reports that Team17's pinned notice on the Steam forum, posted September 23, says the team identified a compatibility problem with KB5124010 and gives uninstalling it as the only current workaround. In Battlefield 6's case, players report the game closing 15 to 30 seconds after launch with no error message, and it stops happening when the update is removed.

On Microsoft's side, TweakTown said there is no reported Microsoft acknowledgment or released fix for the KB5124010 game crashes. The research for this piece found no Microsoft correction as of October 2, 2026.

So Ubisoft is now at least the second studio to point at the same optional update. Several unrelated games built on different engines and protected by different anti-cheat systems are hitting the same wall. That points to a Windows-side change rather than a bug in any one game, though "points to" is not "proves."

Section summary: Siege joins Battlefield 6 and WARDOGS on the list of anti-cheat-protected games reportedly crashing on KB5124010. Microsoft hasn't publicly acknowledged the problem.

The two KBs are not the same kind of update​

KB5124010: the September optional preview​

Microsoft's Windows 11 release information lists KB5124010 as the September 2026 "D" release, the optional fourth-week non-security preview. It came out on September 22 and has a separate build for each version:

The build in Ubisoft's message, 26200.9550, is the 25H2 build. If you're on 24H2 or 26H2 you'll see the same KB number with a different build number, and Ubisoft's advice still seems to apply. The FPS Review notes that Microsoft notes it is the final preview update for 24H2.

KB5121794: the Windows 11 26H2 enablement package​

This one is not a quality patch. Microsoft Support describes KB5121794 as the feature update to Windows 11 version 26H2, delivered as an enablement package. Versions 24H2, 25H2, and 26H2 share a common core OS with an identical set of system files. The 26H2 features already sit dormant on 24H2 and 25H2 machines, and the enablement package is the small "master switch" that turns them on with a single restart. Microsoft lists 26H2 as generally available from September 29, 2026.

Microsoft also lists KB5124010, or a later cumulative update, as a required prerequisite for KB5121794. There's a small inconsistency in Microsoft's own pages here. The KB5121794 article gives the prerequisite build as "26100.9546," while the release information table lists KB5124010 as 26100.9550, 26200.9550, and 26300.9550. The discrepancy is Microsoft's, and it doesn't change the practical advice.

Why this matters for the workaround​

These two packages are linked. KB5121794 needs KB5124010 underneath it, which is presumably why Ubisoft lists both. The consequence is that uninstalling KB5121794 may roll your PC back from 26H2 to whichever version you were on before (24H2 or 25H2), not just remove a minor patch. Neither Ubisoft nor Microsoft has spelled out exactly how this rollback behaves for this incident, so plan on the possibility that you lose 26H2 until you reinstall it.

One more detail for 24H2 users: Microsoft's release information lists the end of updates for Windows 11 24H2 Home, Pro, Pro Education, and Pro for Workstations as October 13, 2026. If you rolled back to 24H2 on one of those editions, you won't want to stay there long.

Section summary: KB5124010 is an optional preview cumulative update. KB5121794 is the switch that turns on Windows 11 26H2. Only remove what you actually have, and understand that removing KB5121794 likely undoes the 26H2 upgrade.

Step-by-step: check first, then uninstall​

Step 1: Find out which updates you have​

1. Press Win + R, type winver, and press Enter. As BottleneckPC notes, reading the build number: 26100.9550 or 26200.9550 means you have it. A build of 26300.9550 means you're on 26H2 with KB5124010.
2. Open Settings > Windows Update > Update history. The FPS Review suggests you look for "2026-09 Preview Update (KB5124010)."
3. Check whether KB5121794 or a Windows 11 version 26H2 feature update appears in the history.

If neither KB is listed, this workaround doesn't apply to you, and your Siege crash has some other cause.

Step 2: Uninstall the relevant update(s)​

This is the Settings path from Insider Gaming's report:

1. Open Settings.
2. Click Windows Update.
3. Select Update history.
4. Scroll to the bottom and click Uninstall updates.
5. Find the listed update(s) (KB5124010 and/or KB5121794, whichever you have) and click Uninstall.
6. Restart your PC.

Ubisoft's statement doesn't say which order to remove them in. Since KB5121794 depends on KB5124010, removing the enablement package first is the logical sequence, but neither company has documented that.

Step 3: Stop Windows from reinstalling them​

Optional preview updates don't normally install unless you opt in. Still, BottleneckPC recommends that after uninstalling you restart, pause updates, and turn off the automatic latest-updates toggle. Pausing is a short-term measure. Don't forget to unpause once a fix is out, because the October Patch Tuesday security update is on the way.

What success looks like​

After the restart, winver should show a build lower than .9550, and Siege should launch normally. If it still crashes, KB5124010 probably wasn't the cause on your machine.

Common failure points​

• Uninstall option missing or failing: Windows doesn't always offer every package for removal. No supported alternative rollback method has been verified for this incident. Don't improvise with risky commands or by disabling security features like Memory Integrity. Contact Ubisoft Support or Microsoft Support instead.
• Still crashing after removal: Treat it as a separate problem. Siege has had other, unrelated crash causes. For example, Intel has an older support article for random mid-game crashes with Arc GPUs, where the fix was to start the game using the Vulkan API instead of DirectX 11. That's a different symptom on specific hardware, not a fix for this issue.
• Driver rollback temptation: Some support replies suggest rolling back graphics drivers alongside the update. As BottleneckPC puts it, if the game worked before the update installed and stopped after, the driver did not change.

Section summary: Check your build first, remove only the KBs you actually have, pause updates for a short time, and treat any crash that survives the rollback as a separate problem.

Analysis: the cost of opting into previews​

The optional preview releases exist so that people who opt in can test fixes before they go mainstream. The FPS Review summed it up with the line that optional preview updates are Microsoft's beta channel wearing a production badge. That's harsh, but it isn't wrong. Kernel-level anti-cheat runs very close to the operating system core, so even subtle OS changes can break it.

In fairness to Microsoft, these are optional updates, and gamers who installed them did opt in, whether or not they meant to. Ubisoft's diagnosis also hasn't been corroborated by Microsoft. Still, with Battlefield 6, WARDOGS, and now Siege all reportedly affected, it's getting hard to call this a coincidence.

There is a reasonable chance this sorts itself out with the next monthly release. The FPS Review expects that fixes baked into KB5124010 will arrive in October's cumulative update anyway, hopefully without the crash to desktop. Neither Microsoft nor Ubisoft has confirmed that October's update will fix the conflict, so check Ubisoft's official Rainbow Six channels before reinstalling anything.

For competitive players, the practical lesson is simple. Skip optional preview updates on the machine you rely on for ranked matches. Let the people who opt into previews find the problems, and pick up the fixes on Patch Tuesday a few weeks later.

Bottom line: If Rainbow Six Siege started crashing at launch after you installed Windows 11 build .9550 or the 26H2 enablement package, Ubisoft's temporary workaround is to remove KB5124010 and/or KB5121794. Check what you actually have before removing anything, expect that removing KB5121794 may undo the 26H2 upgrade, and resume updates once Ubisoft or Microsoft confirms a fix.