Hello,
Please post your sni configuration.
run this :
cat /etc/dovecot/sni.conf
Check if all entries are in this format :
1 local_name "mail.domain.com webmail.domain.com" {
2 ssl_cert = </etc/pki/tls/certs/domain.com.bundle
3 ssl_key = </etc/pki/tls/private/domain.com.key
4 }