First off, please don't tell me to reinstall the OS.
Install another OS instead. Like Linux.You're not really supposed to "Switch Users". non-server Windows versions are really only designed to reliably handle one user at a time.
One thing you can try, at the welcome screen is hitting the three finger salute (Ctrl-Alt-Delete) twice, which should give you a propper login prompt.
Have you seen this bug?
No. But then again, I don't have more than one user on my system.