Wow! Many thanks to you all for your quick responses - answers below:
Alarm never serviced, other than battery replacements by myself.
Yes, error definitely seems to be with remote keypad as the base unit mentions it directly.
Distance from keypad to box is approx 10 feet. The box is in a wooden draw in the front room and the keypad is in the adjacent porch (with a wall in between).
I suspect battery is old, so unless you advise otherwise (based on my responses above), I'll try a battery change.
Thanks again,
Jonathan