A growing number of user reports online suggest some of Apple’s M1 Mac models are susceptible to a Fast User Switching bug that spontaneously activates the screensaver and leaves the user unable to dismiss it.
In macOS Big Sur, Fast User Switching allows users to quickly switch between user accounts without having to completely log out. Based on posts in the MacRumors forums, Apple Community forums, and on Reddit, however, a screensaver can sometimes take over the screen of Apple Silicon Macs without notice, rendering the desktop workspace completely unavailable.
While the screensaver appears on the screen, the mouse pointer is still on top and moving, but nothing else works, and users have to close and re-open the lid on their MacBook Air or MacBook Pro, or light press the Power/Touch ID key or use the Alt-Command-Q key combination to get back to the login page.
MacRumors forum member drmacnut:
Brand new Mac mini M1, 2nd day using it, Big Sur macOS 11.1 (20C69) with screensaver and screen sleep set to 20 mins. I was actively editing some Calendar entries when all of a sudden the same exact thing happened (screensaver on, user locked out). I ended up using Remote Access from my MBP to get in and log out the user on the Mini. The Mac mini then jumped to the login screen and it’s working fine again now. Very weird.
Read more at MacRumors.com