A customer (with S4) had to DNS reverse zones 16.172.in-addr.arpa. 0.16.172.in-addr.arpa. Somethings™ was very unhappy about that and DNS reverse resolving no longer worked. The UMC (or the disgnostics module) should warn about one zone being a prefix of another.
This issue has been filled against UCS 4.1. The maintenance with bug and security fixes for UCS 4.1 has ended on 5st of April 2018. Customers still on UCS 4.1 are encouraged to update to UCS 4.3. Please contact your partner or Univention for any questions. If this issue still occurs in newer UCS versions, please use "Clone this bug" or simply reopen the issue. In this case please provide detailed information on how this issue is affecting you.