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."

Errors

And inside of the server, you'll find corresponding mentions of "Empty startup greeting", "Empty uri setting", or similar within log files located in either of the directories mentioned below. Which directory you'll find the logs in depends on what backend Roundcube is using, SQLite or MySQL.

This directory contains the log files for Roundcube Webmail with an SQLite backend:

/home/username/logs/roundcube/

This directory contains the log files for Roundcube Webmail with a MySQL backend:

/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. You might need to restore a default configuration or template file if necessary, then rebuild dovecot.conf and finally 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/dovecot2.3/main.default

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

/scripts/builddovecotconf 
/scripts/restartsrv_dovecot

Reference the following documentation regarding the Mailserver Configuration interface and Dovecot configuration: https://docs.cpanel.net/whm/service-configuration/mailserver-configuration/