Access to router GUI not possible

I have a Pi hole running as a Docker container.
The raspberry has the router static as DNS Server. The pihole container has the router too as DNS server. On my router (FritzBox) I have entered the IP of the Pi as DNS server, which is distributed to all clients via DHCP. If I now restart the Pi container or the Raspberry, I can no longer get to the GUI of my router. All other GUIs in the network are accessible. Also, the Pi hole and the router are working properly.

If I now switch off the Pi and restart the router, I can access the GUI. If I then change the DNS server from the Pi back to the router and restart the Pi, everything works again. Then I can change the DNS server back to the Pi and everything works again.

Does anyone have any ideas/suggestions/tips for me?

Thank you.

Closing as duplicate of