Thanks for your help!
This is a list of all files and folders with ActiveRecord in the name:
./Services/ActiveRecord/class.ActiveRecord.php
./Services/ActiveRecord/class.ActiveRecordList.php
./Services/ActiveRecord
./Services/ADT/classes/ActiveRecord/class.ilADTActiveRecord.php
./Services/ADT/classes/ActiveRecord/class.ilADTActiveRecordByType.php
./Services/ADT/classes/ActiveRecord/class.ilADTActiveRecordWrapper.php
./Services/ADT/classes/ActiveRecord
./Services/ADT/classes/Bridges/class.ilADTActiveRecordBridge.php
./Services/ADT/classes/Types/Group/class.ilADTGroupActiveRecordBridge.php
./Services/ADT/classes/Types/Text/class.ilADTTextActiveRecordBridge.php
The search for symbolic links did not yield any. I'll check and repair the file system later today and if that does not lead to anything, I'll contact you for a hands-on session.