I would like to use 2 different Skype accounts on different 2 devices (PC / XBoxOne) at the same time. Can there be any problems because of the ports or?
This works without problems.
You can also watch two YouTube videos simultaneously on different devices in the same network.
It communicates over the same port, but that does not mean that using a Skype instance blocks the entire port.
For the XBox you have to unlock ports to have an open NAT. Strict NAT can lead to connection problems. And with 2 XBoxes in a network, one usually has a moderate NAT. That's why I thought that it could possibly cause problems with Skype.
My girlfriend had complained about a jerky transmission while using Skype at the same time. I hope it was the Wlan Repeater fault, which I have now replaced by a powerline adapter.