[files_antivirus] Fatal error: Not Checked. No matching rule for exit code -1 / What to do?

Hello,

Nextcloud logs lots of errors like:
β€œ[files_antivirus] Fatal: Not Checked. No matching rule for exit code -1. Please check antivirus rules configuration. File: ### …”.

What do I have to configure where so that these error messages no longer appear?

Regards, kallle