Did some searching and reckon it might be a battery (CR2477 form an earlier post) But just in case it could be something else I'm going over symptoms here.
1 The board was turned on to find all submaster lights off and contents gone!
2 A backup disk was read in but this did not inhabit the Subs.
3 The system clock is showing correct Date, Time etc.
4 A Sub was programmed and the board turned off for just a minute or so. When it came back up that sub WAS inhabited (green light). Good news. But
5 After leaving off for 5 minutes the subs were again gone on startup.
Sooooooooooo!, No master clock problem but one of a failure to hold on to or load Sub data. (probably cues as well but didn't test them)
The bad news; this board is probably about 20 years old!