When navigating to a page with a Custom Form Part, you see a runtime error. If your organization has a custom error page, you may see the custom error page in place of the runtime error. You may also see a Server Error.
When looking at the testconfig page (yourdomain/testconfig.aspx), there is an error on the Custom Form Part
Testconfig looks like this:
Custom Form Part: Error
The custom form part is a deprecated part since version 6.45. We no longer support it due to the addition of enhancements in Common Forms. Recreate this form as a Common Form.