This also happened to me today Colin. Luckily it was a warm-down rather than the HM that I had just rowed. I hardly ever use ErgData because I use ErgZone most of the time. But I just wanted a "Just Row" session so switched to ErgData after the HM. As you say, the app was working and then the spinning circle started at the end of the session. But no entry in the log on the app or C2 site. It was warning me that the firmware was out of date, so I am updating that now in case that was part of the issue. the PM5 wouldn't let me insert the USB to start with because battery power was too low. Apparently 48% isn't enough to power a USB stick. Really??
I'd also forgotten how much I dislike the C2 Windows Utility Programme. Thank heavens things have moved on.