I tried to find a solution, because this seems to be a very interesting problem. But after trying to see why you are getting the prominent "pam_systemd(plesk_resctrl:session): Failed to create session: Maximum number of sessions (8192) reached, refusing further sessions." I am leaning towards thinking that this does not necessarily have anything to do with the issue. Instead, I only came up with this:
- The time given in journalctl -xe output does not match the times mentioned in your other log. The log does not represent the error situation.
Instead, maybe looking into /var/log/messages for the time when you try to start Dovecot will yield better data for analyzing this.