Roundcube error: Connection to storage server failed

Symptoms

The Webmail login page shows a "Connection to storage server failed" message, and an error similar to the following can be found in the maillog.

Aug 29 21:55:01 23-105-175-248 dovecot[493135]: imap(493949): Error: Failed to initialize quota: Invalid quota root quota2: Invalid rule *:bytes=unlimitedk: Invalid bytes limit: unlimitedk

Description

The issue occurs when cPanel's user cache becomes corrupt. To correct the issue, the cache needs to be rebuilt.

Workaround

1. Access the server's command line as the 'root' user via SSH or "Terminal" in WHM.
2. Move the cPanel users.cache folder out of the way.

   mv -v /var/cpanel/users.cache{,.$(date +%s)}

3. Restart Dovecot.

   /usr/local/cpanel/scripts/restartsrv_dovecot

4. Restart the cPanel service.

   /usr/local/cpanel/scripts/restartsrv_cpsrvd