Navigate to Configuration > System Settings > General Settings (prior to WHMCS 8.0, Setup > General Settings).

Select the Other tab and then scroll to the bottom of the page.

Check to enable Display Errors and hit Save.

Now re-perform the steps that previously were creating a blank page or Oops message. This time instead of a blank page you should see an error message indicating what is causing the issue.

Once you are done, be sure to disable Display Errors again.

If you're unable to access the administration area, there is a manual configuration file flag option that you can add to  the configuration.php file in the WHMCS root directory to enable error reporting.

Edit the configuration.php file and add the following line at the very end of the file:

Upload the configuration.php file.

Now re-perform the steps that previously were creating a blank page or Oops message. This time instead of a blank page you should see an error message indicating what is causing the issue.

Once you are done, be sure to remove the display_errors flag option.