On further investigation I found that error 2 is bad data in the backup. I opened up the text files and found that in every backup I've done... The is a huge block of FFFFFFFFFFFFFFFFFFF in the files. I suspect this is the issue.
I've tried another pc, a different serial port, cleared the desk, saved immediately after then still found the big block of FFFFFFFFFFFFFFFFFFF in the backup file of a blank desk.
I also fed the desk the latest firmware again, and the firmware *updated* fine so the problem is not when loading, or the serial side of things, I think I have a fault when the desk is saving the show to the pc.
Very annoying as I have over 50 hours of programming time wasted.
I don't spose someone can have a look at their txt file and let me know if there's a big block of FFFFFFFFFFFFFFFFFFF near the end of the file please?