HD3D
I just added some basic tests to the extension to check for this problem, but everything seems ok so far: https://github.com/FriendsOfFlarum/default-group/actions/runs/6402803526/job/17380077264
Your stack trace shows that the failure is coming from this line:
$defaultGroup = Group::findOrFail($this->settings->get('fof-default-group.group'));
but seeing as you do't have that setting in your database, the extension will fall back to it's default:
(new Extend\Settings())
->default('fof-default-group.group', 3),
3 is by default the Member group.
This leads me to believe that the problem lies in your groups table. Can you check that this table contains the member group with an id of 3?