Hi Philip,
You need to:
1 - Create a screen variable of type Status Identifier, let's call it StatusId;
2 - Set the variable of the Button Group widget to be StatusId and define a screen action on the OnChange event:
3 - Set the value of each button item to Entities.Status.Label except the first one "Alle tasks" that you should set to NullIdentifier() (which would not be filtering)
4 - On your query GetBestehendeTasks, add a filter StatusId = NullIdentifier() or Status.Id = StatusId (this will make your tasks be filtered, except on the All Tasks option where no filter will be applied)
5 - Refresh your query on the screen action of the OnChange event you set on step 2.

And that's it, hope it helps.
Regards,
João