I messed up my registry using SubInAcl.
When I tried switching user, Windows froze just before displaying login screen.
Then I rebooted the PC, but Windows froze again just before displaying login screen.
When windows is frozen, if I press alt+tab, I can switch between "Log on to windows" and "Windows logon". Choosing "Log on to windows" does nothing. Choosing "Windows logon" does nothing, except making alt+tab stop working.
Booting into safe mode or last known good configuration still produces the same problem.