Error downloading mvn:org.openhab.binding/org.openhab.binding.exec/2.4.0

Hi everybody,
I run openhab 2.4 on a raspberry pi behind a fritz box, which is connected to another router.
If I let the fritzbox run on IP-Client-Mode, everything works just fine.
However, as soon as I set up the fritzbox to establish its own internet connection I get the error above and Paper UI is not showing on the dashboard anymore (because it can’t be loaded any more either). None of the addons can be downloaded. I have also tried to completely expose the raspberry pi because I suspected the fritzbox firewall is the problem, but that doesn’t help either.
Does anybody have an idea?
Best regards

My idea is that it sounds like a fritzbox issue so you would get better answers from a fritzbox forum.