I tried to do an actual firmware upgrade, via the printer’s own FTP connectivity, but on doing the put of the file to the printer it just sat there with no activity and no progress.
The “solution” – if you can call it that – is to clear and hide the error.
Power on the printer
When the memory count begins, press and hold the stop (red X) button. Keep the button depressed (approx. 10 seconds) until all three control panel lights flash and then remain solidly on.
Press OK and then the Menu button
Scroll down until you see the Clear All RFU Errors option
Press OK twice (or until the cursor is highlighting the C in the word Clear)
Scroll down until you see the Set Runmode to Manual option
Press OK once (or until the cursor is highlighting the R in the word Runmode)
Scroll down until you see the Set RFU Error Off option
Press OK twice (or until the cursor is highlighting the S in the word Set)
Turn the printer off, wait 30 seconds before powering back up again – the error should now have disappeared