Neither Roundcube nor Horde load for any email addresses on a cPanel account

Symptoms

The Roundcube and Horde webmail clients will not load for any email addresses on a cPanel account. The webmail clients will load for most email addresses on the server, other than those associated with the affected cPanel user.

The error when accessing Roundcube indicates a database connectivity issue:

DATABASE ERROR: CONNECTION FAILED!
Unable to connect to the database!
Please contact your server-administrator.

Accessing Horde results in a 'too many redirects' error in the web browser.

The cPanel PHP-FPM error log at /usr/local/cpanel/logs/php-fpm/error.log shows the following:

ERROR: epoll: unable to add fd ###
ERROR: epoll: unable to add fd ###
ERROR: epoll: unable to add fd ###

Description 

While the root cause of the issue is unknown, the 'unable to add fd' errors in the above PHP-FPM error log result in the service being unable to load PHP-FPM pools for affected users.

Workaround

A hard restart of cpsrvd and cpanel_php_fpm should resolve the issue:

/usr/local/cpanel/scripts/restartsrv_cpanel_php_fpm --hard

/usr/local/cpanel/scripts/restartsrv_cpsrvd --hard