This article covers the issue when clicking "Check Email" from cPanel gets stuck spinning similar to the screenshot below. One possible cause is a limit in the cpanel_php_fpm service.
The systems administrator of the server can confirm the issue in the cpanel_php_fpm error log:
# tail /usr/local/cpanel/logs/php-fpm/error.log
NOTICE: fpm is running, pid 1291
NOTICE: ready to handle connections
WARNING: [pool user_username] server reached max_children setting (25),
consider raising it
This issue can be worked around by either closing running cPanel PHP-FPM processes under the user (like webmail or PHPMyAdmin), or disabling cpanel_php_fpm.
Alternatively, the max_children limit can temporarily be raised in one of these files:
Note, changes made to the user's configuration will not be retained on reboots, and the template may be overwritten upon cPanel updates. Disabling FPM for cPanel services would be a better alternative.
There is an open feature request that you can vote on and submit any feedback that you think our developers might find helpful.