I'm having the same issue. I can only access the content server on the device which is running calibre (desktop PC), using the address provided in the user manual for this purpose (
http://127.0.0.1:8080 - I have no clue if these numbers are unique to me or the same for all users).
Previously I've been able to access the content server from other devices on my home network. (My ISP doesn't allow port-forwarding so I have no experience connecting from a different network.)
Now, when I try to access the server from a different device on the same network (android phone), by using the IP address listed by the "stop content server" button, I get ERR_CONNECTION_TIMED_OUT messages.
The IP address that is populated in the user manual has a different host ID, and trying that one results in the message ERR_ADDRESS_UNREACHABLE.
Not sure if this matters or not, but clicking "Visit Content Server in browser" under the "Connect/Share" button no longer links to
http://127.0.0.1:8080/ but instead
http://[::1]:8080/. It appears to work the same.
In case it isn't obvious, I know very little about this but I can follow directions.