As always, Microsoft released new updated for all supported versions of Windows 10 on the second Tuesday of the month, a day affectionately referred to as Patch Tuesday. Unfortunately, these updates ended up being problematic in a big way, causing a bug that left users with a blue screen of death (BSOD) when they tried to use a printer.
After offering a temporary workaround over the weekend, the fix is now here in the form of an out-of-band cumulative update. In these updates, there is just one fix, which "addresses an issue that might cause a blue screen when attempting to print to certain printers using some apps and might generate the error, APC_INDEX_MISMATCH." In other words, this update is pretty straightforward.
It should be available for anyone affected - specifically those on Windows 10 versions 1803, 1809, 1909, 2004, 20H2, and Insiders on 21H1 - but it's not going to be installed automatically. If you go to Windows Update and check for updates, you'll see that there's a new optional update, which will look just like a C week update. That's the one that you'll need to fix the issue.
The regular C and D week updates will arrive later on this month, and Microsoft has already said that those are going to remove the old Edge browser, so we know they're coming. Any future cumulative updates will also include this fix.