No connection - only within a specific network / internet provider

Hi, I use Teamviewer for long time and it works fine everywhere. But I have a new internet provider "OR-NETWORK GMBH" and a Fritzbox as router where Teamviewer tells me "NOT READY. PLEASE CHECK YOUR INTERNET CONNECTION".

I have 3 computers Mac, Windows and Ubuntu that run well at other internet providers / networks but not within this new one. All other internet applications (Browser/Skype/etc) run well. Where can I look for the issue? Any idea about Fritzbox settings maybe?