No idea at all, sorry.
- Does it work if you set "$spam_quarantine_method = undef;" in /etc/amavis/conf.d/50-user? (Restarting Amavisd service is required after you added this parameter.)
- Since the SPAM is bypassed and delivered to user mailbox, it's SAFE to delete quarantined one under /var/lib/amavis/virusmails/. File name of quarantined spam starts with 'spam-'.