What serial number does Mosaic report in the Device Settings section? (it’s under Support or Status, can’t remember which and I’m not home at the moment). What does it report on the front panel display? If they are the same there is no issue so I wouldn’t worry about it.
It seems that you do have a serial number. I am guessing that other than this issue it is all functioning correctly.So it looks like a software oddity.I have no idea why so perhaps @Phil can assist you?
The good news is that your unit does actually have a serial number installed, contrary to the name it has given itself.
Could you perform a ‘RESET INPUT NAMES’ via the ‘Config’ section of the menu please? It should change the unit and it’s input’s ‘names’ back to their default factory value. Which, in your case, should be ‘dCS Rossini DAC’.