If you are managing a Partner Portal and one of the users contacts you with an Internal Error message that gives you zero amount of information to look for, here is one thing that you should keep on your checklist.

Any account owner in Salesforce that is attached to an active Partner Account needs to have a role on their user record.

Other items to keep in mind if your users are having trouble accessing would be:

1. Make sure the Owner of the Account in Salesforce is an ACTIVE user
2. Make sure that the self-registration pages are accessible to guests and that you have guest users type enabled
3. Make sure you have roles enabled for the users that will be self-registering
4. Make sure you have no automation in place that could cause them to fail self-registration
5. Make sure that the Partner Account is actually enabled for the Partner Portal