Windows 10's latest cumulative update is coming off the rails due to a security update that's installed alongside the Patch Tuesday upgrade for January.
Bleeping Computer reports that the KB5034441 security update is failing to install with an '0x80070643' error code for some Windows 10 users. That update patches up a security flaw, namely a BitLocker encryption bypass that could let an attacker access encrypted data (leveraged via the Windows Recovery Environment).
After KB5034441 failing and the PC rebooting, users are presented with a message informing them that there were problems with installing updates, and the OS would try again later.
Microsoft has addressed the issue already, though, and identified the problem - namely that some PCs don't have a recovery partition that is big enough to complete the update.
In this case, you should get the following error: Windows Recovery Environment servicing failed. (CBS_E_INSUFFICIENT_DISK_SPACE).
However, there's a problem with the error handling which means that Windows 10 is instead producing the '0x80070643' error instead, which doesn't give you any clue as to what's gone wrong.
The upshot is that you need to manually resize your recovery partition to allow for the security update to install: just follow Microsoft's instructions here.
That said, Bleeping Computer cites a report from a reader stating that a large partition didn't cure the installation failure. So, you may want to wait for Microsoft to investigate further, and perhaps implement an automatic (less clunky) fix. Especially as the vulnerability in BitLocker requires the attacker to have physical access to your PC, so isn't too much of a concern for a good many folks in that respect.
Meanwhile, regarding Windows 11's new cumulative update (KB5034123), there are a couple of scattered reports of problems here. One is an installation failure in a similar vein to the above Windows 10 glitch - and we're seeing quite a few of these of late (in fact, this is a persistent problem).
The other is a report of the update causing graphical corruption and weird screen flashes.
Of course, both of those could be isolated incidents that are ultimately faults with specific PC configurations. We'll certainly be keeping an eye out for other reports along these lines in the near future, though.
The Windows 11 update doesn't do much anyway, mind, although one new (minor) feature is the addition of dynamic, interactive weather updates for the lock screen.
As for Windows 10's cumulative update for January, that doesn't do anything feature-wise, but does fix security flaws and some bugs including one that causes the PC to shut down (in an admittedly niche scenario with using a smart card).