Hi @M_Polak,
That screen typically means that your Core could not login to your Roon account properly.
If the issue re-occurs, I would check the DNS servers on your router and try to use Cloudflare DNS, Quad9 or Google DNS.
You might also want to look into using a consumer-grade router like Netgear/TP-Link/ASUS/etc. instead of a generic router for better performance, our Networking Best Practices Guide also provides some great suggestions.