Some HTTP 500 errors are caused by the .htaccess file, it may help to temorarily rename it or reupload the file from backup.
Possibly wrong PHP version or missing PHP modules is not the cause.

In my case of old WHMCS 6, the blank page with error 500 has been caused by the MySQL strict mode enabled.
This setting enforces stricter data validation, which can interfere with applications like WHMCS. By disabling strict mode, we were able to restore compatibility, ensuring that WHMCS operates as intended.