Unable to log into Webmail on Plesk with Dovecot: Connection to storage server failed

Symptoms

• Unable to log in to Webmail with the following error:

  Connection to storage server failed.

• Dovecot is installed on the server;

• The following entries can be found in /var/log/maillog:

  dovecot: imap-login: Warning: Auth process not responding, delayed sending initial response (greeting): user=<>, rip=::1, lip=::1, TLS, session=<KtEFriOGgq4AAAAAAAAAAAAAAAAAAAAB>
  dovecot: imap-login: Error: Timeout waiting for handshake from auth server. my pid=31113, input bytes=0
  dovecot: imap-login: Disconnected: Auth process broken (disconnected before auth was ready, waited 30 secs): user=<>, rip=::1, lip=::1, TLS, session=<KtEFriOGgq4AAAAAAAAAAAAAAAAAAAAB>

• The following entries can be found in /var/log/plesk-roundcube/errors:

  IMAP Error: Login failed for jdoe@example.com from 203.0.113.2(X-Real-IP:203.0.113.2). Wrong startup greeting (ssl://localhost:993): * BYE Disconnected: Auth process broken in /usr/share/psa-roundcube/program/lib/Roundcube/rcube_imap.php on line 196 (POST /?_task=login&_action=login)

• Valid certificate is used installed on IMAP service:

  # openssl s_client -showcerts -connect localhost:993
  CONNECTED(00000003)
  depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
  verify return:1

Cause

Corrupted Dovecot packages.

Resolution

Warning: switching from Dovecot to Courier and vice versa may result in all the messages being re-downloaded by Mail Client Software (Outlook, Thunderbird, etc.)

1. Connect to the server via SSH

2. Create a backup of Dovecot folder:

   # cp -R /etc/dovecot /root/dovecot/

3. Log into Plesk

4. Go to Tools & Settings > Updates & Upgrades.

5. Install Courier service to remove Dovecot.

6. Remove existing Dovecot configs:

   # rm -r /etc/dovecot

7. Install Dovecot again.

Additional information

Are there any issues when changing the IMAP/POP3 server in Plesk from Dovecot to Courier?