You haven't given us many details, like your computer specs and what Update you installed so, the answer may be vague as well.
If you can still login to Windows as an Admin in one of your other accounts, you can uninstall the update.
And/Or update the Video driver from the manufacturer
When I try to login to one of my accounts, I get the screen in attachment with the round item rotating continuously.
The background looks like the one of Accessibility.
Is there any way I can identify the process to (try to) deactivate it?
Yes, same machine, and still no solution.
The update occurred mid-August, not found any way to revert.
Since some other accounts work fine, I guess the drivers must be OK.
From one suggestion, I created a new admin account, and got a similar screen when Windows asked for some stuff about accessibility.
Would there be one item that lies behind that?
If necessary, I could restart with Linux and (temporarily) rename the culprit.
Note: Your antivirus program may report FRST incorrectly as an infection. If so, disable the real-time protection when downloading and running FRST.
Right-click to run the tool as administrator. When the tool opens click Yes to disclaimer.
Note: Ensure that the Addition.txt check box is checked at the bottom of the form within the Optional Scan area.
Press the Scan button.
Please wait for the tool to finish. It will produce two logfiles called FRST.txt and Addition.txt in the same directory the tool is run from (which should be the desktop)
Post the logfiles FRST.txt and Addition.txt as attachment in your next reply.