Can't open my.nextdns.io when mullvad is active

I'm using the mullvad add-on in my Tailscale network, and I'm also using nextdns as global name servers for this network.

When I select a mullvad node as exit node, I cannot open https://my.nextdns.io. I get the error that the address is invalid. Visiting https://test.nextdns.io gives me a JSON-output with status "ok", so I guess nextdns is filtering my traffic. But I also want to be able to open the configuration panel.

Does anyone experience the same issue?