Wow… I already thought I’m stupid as I couldn’t find the reason why just a few users of a certain location were not able to log in. During the previous weeks I made my personal deep dive (at least for my knowledge) into Nextcloud CLI and database. However nothing helped - except this Google search result 
Adding the corresponding DC to “logon to” or removing all computers from “logon to” fixed the “issue”. Thank you.
Just to improve Google search results even more, I add the current event which appears in the log:
[user_ldap] Warning: Bind failed: 49: Invalid credentials