Hi @TheFritz89, welcome to the Sonos Community!

There are a few steps we can perform on the community regarding the error you’re receiving, please attempt the following and try to browse your Plex library after each step:

1. Remove and re-add Plex to re-authorize the account on Sonos.
2. Check if other music services work, such as radio, to ensure the controller isn’t experiencing a DNS failure.
3. Attempt to log into app.plex.tv to confirm your account and library are accessible.
4. Confirm that a controller is able to access and browse the server outside of the local network. Such as a phone or tablet that is
   not connected to the WiFi (running on 4g/5g)
5. If you can log in, ensure that Remote Access is enabled on the Plex server. This is found in Settings > Server > Remote Access. 

If you aren’t able to enable remote access or you receive an error when doing so, then it would be best to reach out to Plex for further assistance in enabling that setting. However, if Remote Access is enabled, and the steps above don’t resolve the issue, then I’d recommend reaching out to our support team for further assistance as they have the tools necessary to troubleshoot this issue with you.

I hope this information helps!