Dag Nygren via dovecot:

One more obvious line from the log:

dovecot[26621]: auth: Error: LDAP: Connection lost to LDAP server,
reconnecting

usually reconnecting works. If it doesn't for you, it's probably not
dovecot's fault.

I suggest to inspect openldap logs. Try to stop slapd and start
(ideally: same environment and parameter) appending "-d 320". That "-d 320" prevent slapd becoming a daemon and
flood stdout with logs. So it is really helpful if there are no other LDAP clients when you
run slapd this way.

You should see dovecot reconnecting and maybe a reason why that fail.

Andreas