I'm not sure if this is exactly the same as https://www.virtualmin.com/node/2941 it doesn't seem to directly related to a timeout, but a program failure is causing the same result of mail being lost.
In this case there's an unknown issue that caused clam or the wrapper to be killed but, the filter shouldn't default to marking as a virus and sending to /dev/null or perhaps should do some kind of quarantine instead especially since there's no bounce, no warning (other than in procmail.log after the fact) and no way to recover the mail. For now I've disabled virus scanning as a work around.
procmail: Program failure (-15) of "/etc/webmin/virtual-server/clam-wrapper.pl"
Time:* From:* To:* User:* Size:* Dest:/dev/null Mode:Virus
sh: line 1: 1203 Killed