ENDIAN Firewall - run_av (ClamAV-clamd) FAILED - unexpected exit 255, output=""

Since a week or so no mail was scanned by ENDIAN, found error(s) in log files:

Quote

Jun 7 17:46:19 mail amavis[10665]: (10665-01) (!)run_av (ClamAV-clamd) FAILED - unexpected exit 255, output=""
Jun 7 17:46:19 mail amavis[10665]: (10665-01) (!)ClamAV-clamd av-scanner FAILED: /usr/local/bin/av-scan unexpected exit 255, output="" at (eval 86) line 596.
Jun 7 17:46:19 mail amavis[10665]: (10665-01) (!!)WARN: all primary virus scanners failed, considering backups
Jun 7 17:46:19 mail amavis[10665]: (10665-01) (!!)TROUBLE in check_mail: virus_scan FAILED: AV: ALL VIRUS SCANNERS FAILED
Jun 7 17:46:19 mail amavis[10665]: (10665-01) (!)PRESERVING EVIDENCE in /var/amavis/amavis-20210607T174617-10665

I found a lot of preserved files in /var/amavis, my question(s):

how can I release those preserved mails?
how can I solve the ClamAV issue?

Thanks
Werner