Webmail Login stuck on "Logging you in ...".

Symptoms

When clicking the Check Email button in cPanel, the login gets stuck spinning, similar to the screenshot below.

An entry similar to the following is also present in /usr/local/cpanel/logs/php-fpm/error.log:

CONFIG_TEXT: WARNING: [pool user_username] server reached max_children setting (25), consider raising it

Cause

This issue occurs when the cpanel_php_fpm service hits the max_children limit. This limit can be temporarily increased per user, or globally. The limit increase is temporary as changes to the cPanel user's configuration file will be lost upon reboot, and changes made to the global template file will be overwritten when cPanel updates.

Resolution

Update the max_children setting for the cpanel_php_fpm service. 

Server reached max_children setting (25), consider raising it (cPanel PHP-FPM)