Hi Peter,
You mentioned rebooting, which is what normally corrects this issue based on other threads. Did you turn the power off, wait to a 10 count, and then turn the power back on? Or more of a restart of the RoonServer via the Web Interface?
If a full shutdown and restart of the machine doens’ work, you might try clearing the Roon cache. First, shut down any attached clients.
From the Web UI on the ROCK server, stop RoonServer. Then go to the \\rock\Data\RoonServer directory and rename the Cache directory to Cache_old. Then restart the RoonServer. Once it is restarted and you see the Cache directory rebuilt, then try logging on with the client.