The Unable to get 'From:' string from SMTP session fix for qmail has now been published by Parallels as a micro update (along with the fix for compatibility with the latest OpenSSL update). Since the Plesk updater does not seem to use RPM packages for the micro updates (bad!), but just replaces some files, the qmail-scanner update trigger does not fire and your qmail-queue file will be overwritten by the micro update, which disables qmail-scanner in the process. Running 'qmail-scanner-reconfigure' won't fix this.

I ended up removing and reinstalling qmail-scanner after the micro update.