Is the PowerPack Registration Form widget not working on your WordPress websites?
This article explains how to resolve the "Elementor Registration Form Widget Not Working" and "Server Error" issues.
Why does the Internal Server Error occur?#
When the post_id parameter is not provided, it produces a 500 Internal Server Error, but the registration form widget still works properly.

How to Fix the Registration Form Widget Issue?#
Step 1: Add the below code to wp-config.php
// Excludes arguments from stack traces generated from exceptions.
@ini_set( 'zend.exception_ignore_args', 0 );

Step 2: Update the Registration Form widget page.
Step 3: Clear Cache
Done!
If the issue persists, please feel free to contact our support team. Hope this helps!