Roundcube Webmail login page says "Connection to storage server failed"

Symptoms

When attempting to login to Roundcube Webmail from cPanel, the interface goes to the Roundcube login page and says, "Connection to storage server failed." Mentions of "Empty startup greeting," "Empty uri setting," or similar may be found in the Roundcube log files.

Please note, the directory in which the logs are located depends on what backend Roundcube uses, SQLite or MySQL. If Roundcube uses an SQLite backend, the log files will be in /home/username/logs/roundcube, where "username" is the cPanel user's username. If Roundcube uses a MySQL backend, the log files will be in /var/cpanel/roundcube/log. 

Description 

This error can occur for many reasons, including too many open IMAP connections, IMAP not being enabled, or a failing Dovecot authentication process.

Workaround(s)

IMAP:

• Close some of the existing connections by having users log out of their email clients.
• Increase the Maximum Number of Authentication Processes from "WHM » Home » Service Configuration » Mailserver Configuration."
• Verify that IMAP is enabled in "WHM » Home » Service Configuration » Mailserver Configuration."

Dovecot:

• Verify that the Dovecot authentication process is running.
• Verify the Dovecot configuration files exist and have proper syntax. If necessary, you might need to restore a default configuration or template file, then rebuild dovecot.conf and restart the service.

Notes

The Dovecot authentication process is commonly shown as "dovecot/auth" in the output of the following command:

ps -aux | grep dovecot/auth

The default template file is located at the following location:

/var/cpanel/templates/dovecot/main.default

You can rebuild the configuration and restart the Dovecot service with the following commands:

/scripts/builddovecotconf 
/scripts/restartsrv_dovecot

Additional resources

Mailserver Configuration