Dear all, thank you so much for your support.
I did the unattended setup without changing the config file. Seems there have been some incompatibilities with my system. Virus scanners were not installed.
I have to correct my earlier statement about the pam_unix.so. Just like @Wolfgang_S said, it’s in /usr/lib/x86_64-linux-gnu/security/, not in /lib/security/.
The solution mentioned in the other thread seemed a bit shady to me.
Instead of poking in the dark, I decided for a fresh install like @apella12 suggested. Anyways I had to update my Debian.
Cheers