in /var/log/maillog I had many lines like this:
amavis[18486]: (18486-01) (!)run_av (ClamAV-clamd) FAILED - unexpected , output="/var/amavis/tmp/amavis-x/parts: lstat() failed: Permission denied. ERROR\n"
I fixed that adding the clamav user in amavis group with this command
usermod -a -G amavis clamav
I guess it's a bug, isn't?