[Koha] Circulation anomalies

[Mesfin Teshome]

Dear all,
I am facing a strange problem. I am a system/network admin and our librarians at the University are saying Koha is reporting no checked out items for people who have checked out books, or displays 1 or 2 checked out items while there are many books checked out. The librarians claim they have not done anything with the circulation.
When I check the circulation history and modification logs at the Patrons profile, the logs show that the books have either been returned or issued but the librarians swore they have not done that and that the system is doing that by itself. I have tried to look at the Koha-error_log, I see this error message
returns.pl: Use of uninitialized value in hash element at /usr/share/koha/intranet/cgi-bin/circ/returns.pl line 413, <DATA> line 522., referer: http://$libraryurl$/cgi-bin/koha/circ/returns.pl
Is this related to the problem? Where can I start troubleshooting the problem? The librarians are very worried because bad record with circulation could mean loss of books and they are convinced it is a system problem.

We are running Koha version: 3.12.00.000
I will really appreciate your response.
Kind regards,
Mesfin