Turns out it's doesn't happen all the time, like all the best bugs
Rowed a half-marathon today, uploaded to the C2 logbook, then successfully transferred from the USB stick to an Excel Spreadsheet using the C2 Utility (twice). I hadn't changed anything else. I did download the previous version of the C2 Utility (6.99), but didn't install it, so that shouldn't make any difference.
@Matthias: I didn't try opening the C2 Utility as Administrator, but I'll keep that in mind if I have trouble with the spreadsheet function again.