I created a ticket on this a couple of days ago, but can't seem to find it now.
What seems to be happening is that whenever a letsencrypt SSL certificate is renewed, the SSL certificate section of dovecot.conf is getting corrupted.
Up until yesterday (happened three times), it was missing the closing brace for one or more local_name clauses in the dovecot.conf file.
However, last night it was much worse. It's missing closing braces, opening braces, and even local_name declarations.