The title sums it up. The virtual server was deleted, but the references to the deleted site's SSL cert were not removed from
/etc/dovecot/dovecot.conf. It continued to work fine until I needed to restart Dove for an unrelated reason, then barfed on the errant entries.
I don't need support on this because the problem was obvious and easy to fix (delete the errant entries). It's just a bug report.