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."

And inside of the server, you'll find corresponding mentions of "Empty startup greeting" 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 be caused by too many open IMAP connections. This error may also be caused by IMAP simply not being enabled.

Workaround(s)

• 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

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