FRITZ!Box disconnects internet connection when connected to LAN?

Hello everyone,

My parents received a FritzBox 7590 from Telekom, which also worked flawlessly for a long time. I recently installed a powerline system at their home and was surprised. As soon as I plug in or unplug a LAN cable, the box disconnects the internet connection, only to re-establish it within about two minutes. Then everything works again until a LAN cable is plugged in or unplugged. The system doesn't seem to care at all which port is plugged in or unplugged. To be honest, I'm a little confused. Has anyone experienced this before?

And a second problem: when my father turns on his PC, the powerline system, which creates a Wi-Fi connection at the other end, seems to lose its internet connection. When the PC is turned off, the connection is restored. I've only requested and retrieved data via remote diagnostics. I suspect an IP conflict, meaning the PC is getting the IP address of one of the powerline systems, even though that shouldn't be the case with DHCP. But maybe someone else has an idea 🙂 The powerline system is a TP-Link TL-WPA4220 set.

Many thanks in advance and kind regards,

Chris