This is probably the root cause. Check your dovecot.conf for the ssl_cert path (and possibly ssl_key, etc.) to see if the files exist where Dovecot thinks they do, and make sure they’re actually in PEM format.
I’m getting vague notions from various problem reports that maybe there’s a situation where some certs that Virtualmin is copying into place aren’t a valid format for Dovecot (and it may be Dovecot version dependent). But, I’m still guessing pretty wildly and I don’t really have enough data…I probably need to see your Dovecot config file (sanitized if you don’t want to show domain names or paths). And, I need you to tell me whether there are actual PEM files in the places Dovecot is looking for them.