Error Message after Registration submit 'Invalid Group ID'

Hi,

I upgraded from Expression Engine 2.x to 3.5.4 now. I’m using PHP 7.0.15 with 3.5.4 currently on my local development system. The production site is still currently running under 2.x.

Many things I got corrected and running. It was not very easy.

But one thing is still remaining. When I fill out the Registration form (/member/register), for new Users, then an error Message appear, which says that I have to choose a valid group.

Message:
group_id: Sie müssen eine gültige Gruppe wählen

Message-id:
invalid_group_id

Until now this was not necessary, so a new member would get the “member” group automatically. Is there an Option I was missing? How could I get more Information about this error and how to avoid it.

Currently it is not possible to register as a new member on my development system. When I got this corrected, then I try to take the system to production.

Any ideas?

Regards Jürgen