Problem statement: problem first appear after update of Webmin+Virtualmin, when starting or restarting Dovecot the error message about not found certificate file. The very same certificate file name is used in the dovecot.conf, in several "local_name" directives, while dovecot itself has its own cert file. (NB: the several directives "local_name" for each virtual server seems to appear as new, after update of Virtualmin -> not sure)
Expected: Dovecot starts normally, no certificate files deleted.
Configuration Note: Automated Let's Encrypt certificates renewal is enabled, 2 months between. The certificate is multi-domain, the Virtual servers report the usage of "Certificte Sharing", with one master server.
Suspected root-cause: misconfiguation, or failure in the Let's Encrypt routines, which makes to delete the certificate, before successfully retrieving the new cert. Suggested to validate routine "apply_letsencrypt_cert_renewals" in /usr/share/webmin/virtual-server/feature-ssl.pl.
Workaround: manually recover/copy the cert files, then disable the auto-renewal of certs.
System: Operating system Debian Linux 9 Webmin version 1.870 Virtualmin version 6.01