Posting here for better visibility:
It seems that something is (still) happening with XFCE on proprietary Nvidia drivers (since [Stable Update] 2023-12-01) that causes problems with session locking and switching
Xfce4 black screen with cursor when switching between users
After 2023-12-01 I can not get in to a timed out session
EDIT: it still happens on Kernel 6.6.5-3