I figured it out! It was a firewall issue.
I use Handsoff and I checked it before i wrote here. There was no "user defined rule" that denied the communication but when i changed to "All rules" the program displayed several rules regarding AppleTV and its IP.
So if anybody has the same problem make sure you see all rules of your firewall and get rid of the AppleTV related rules generated by the program itself. You dont need to update to the latest iOS on the AppleTV!
I hope that I can help some people with this info to fix the problem.