We had the same " Oops! Something went wrong and we couldn't process your request." after the client logged in. Hitting back did allow users to get to client area. We turned Display Errors on in General-Settings>Other. This displayed the error below and we were able to find the Module causing our issue. Just for everyone's info; we removed the module and then installed the latest version but finally decided to just not use it. In our case it was no longer needed.
Fatal error: Call to a member function item() on null in /home/bgnhosti/public_html/modules/servers/cpanelExtended/classes/cPanel/unban/class.cpanelUnbanModule.php on line 213
Oops!
Something went wrong and we couldn't process your request.
Please go back to the previous page and try again.
exception 'Whoops\Exception\ErrorException' with message 'Call to a member function item() on null' in /home/bgnhosti/public_html/modules/servers/cpanelExtended/classes/cPanel/unban/class.cpanelUnbanModule.php:213
Stack trace:
#0 /home/bgnhosti/public_html/vendor/whmcs/whmcs-foundation/lib/Utility/Error/Run.php(0): WHMCS\Utility\Error\Run->handleError(1, 'Call to a membe...', '/home/bgnhosti/...', 213)
#1 [internal function]: WHMCS\Utility\Error\Run->handleShutdown()
#2 {main}