Looks like the same topic like this:
As I already pointed out there, clear your cache and try logging in again.
zammad run rails Cache.clear
I currently don’t see any reason why this should appear.
However, it would be good to know which authentication source you guys are using. Because the steps to reproduce are not clear.
Just before anybody screams:
Zammad will not remove users on it’s own unless one tells it to do so.