Update to UCS 4.4-5 fails with Verification error: Invalid signature: gpgv: keyblock resource '/etc/apt/trusted.gpg': File or directory not found

If the update to UCS 4.4-5 aborts, the file /var/log/univention/updater.log should be checked for the following error:

**** Downloading scripts at Thu Jun 25 11:34:04 2020
Error: Update aborted due to verification error:
Verification error:
 Invalid signature: gpgv: keyblock resource '/etc/apt/trusted.gpg':
  File or directory not found                                     

If the error appears in the logfile, all available errata updates should be installed, before starting the release update to UCS 4.4-5.

This issue will affect all UCS 4.4-4 systems that have a errata level between 605 and 641. Once the server version is UCS 4.4-4 errata 642 and above, a release update to UCS 4.4-5 is possible. The current errata level can be determined by running univention-app info or in UMC by opening the side menu and clicking on Help -> About UMC.

See Bug 51576 for more information