After some packages/data got corrupted, I ran fsck and repaired the packages from the recovery console. As far as I can tell all packages should be whole and apt does not complain. However - I cannot change users' password as root - I get
passwd: permission denied
passwd: password unchanged
I also cannot login, but it might be a possibility that I forgot the pw of this machine/account, but resetting it doesn't work. :-/ I reinstalled all relevant packages and made sure to replace the config files with the standard ones (/etc/pam.d/common-*) and also checked the pam.d files by hand. As far as I can tell they should work, no weid plugins are installed as far as I can tell.
When I check journalctl -xe, it reports
passwd: PAM bad Jump in stack - all info points to this being an issue with pam config files and pam jumping over a row to a 'deny' an important row, for example. But as the config files are the stock ones, with only unix login enabled. ssh server and polkit are/were installed, but even removing their files doesn't fix the bad jump error.
Any ideas? I can upload all the configs on pastebin if required/requested.
Update: I checked various forums and answers here, but no case fit properly. Contrary to other questions/issues, cron is currently not used and crontabs are empty. Unconventional modules like fingerprint scanner access were also never installed.