Once disabling the custom error message you should be getting a different error page, but that's fine, I am pretty sure that the issue you are having is related to the portal version
navigate to Power Platform admin center (microsoft.com)
on the left, go to Environments --> select your environment
and then click on Resources --> Power Pages:

Open your website click on Edit within Site Details, and select "Enable the Portal for Early Upgrade"

this will help getting the most recent version, it might take some time to update the Portal as it would probably do this during off-peak hours.
Alternatively, you can open a support ticket with Microsoft and ask them to push the latest version for your environment, but I doubt that this would be any quicker