Every so often, when I open Mosaic it “sees” the device but says it is type unknown rather than a Rossini Player.
All of the menus are blank, but it can tell which input it is set to and that some “unknown” device is there.
To recover, you need to shut the Rossini Player off and everything is normal again when the RP finishes connecting to the network. No other combination of front panel presses or rebooting the device running the Mosaic will convince the RP to talk to the app again.
It looks like others have seen this too, with the same net workaround - power cycle the RP.