Could you please explain me, why I have in the category "Gäste" 13 users 1 (pic1)
but in the category "Gäste" in the second table 0 users (pic2, I will add this later)
Here is the second picture, because I was unable to attach both of them.
We don't have answer on this question because this is not wpForo core issue. This is a result of incorrect migration configuration. The original Guest usergroup is located under the "Gäste" duplicated usergroup.
If you look at the numbers attentive, you'll see that all your 1694 users are "71G Mitglied" in the second screenshot. So, there are 0 users in "Gäste". You should go to Dashboard > Forums > Dashboard admin page, click on the [Delete all Cache] button, then go back to Forums > usergoup admin page and click on [Sinchronize] button again.
