Oh ok. But why is it not possible for me to mark the others?

Because you should not allow users with "Registered Usergroup" to edit/delete other people topics and posts. This made for admins who don't understand that a simple registered user is not admin and he should not have rights to edit other members topics and posts.

I'm sorry but I have no clue how to explain you. Please ask someone to help you in translating my posts.

Now I understand. But my users told me that they can not edit their posts. What could it be that they can not edit their own posts? Is there something else I have to activate?

There are two more things you should know:

1. There are time limits in Dashboard > Forums > Settings > Topics & Posts admin page. Users cannot edit own topics and posts if the time is passed.

2. If a new post becomes unapproved, the author of this post will not be able to edit own post. Unapproved topics and posts can not be edited until admin approve those.