I got a reply from Support: in summary (and reading between the lines) they say the stats for a print file don’t get re-scanned automatically (in 1.5, I presume). They say the error report was caused by printing the file first time using pre-1.4 firmware (which it was) and then updating to 1.5 and trying to print again.
They say this will be fixed.
They also say (and this is the real news, to me anyway) that a long press on the item in the file list re-scans it, and should make it work. I’m no longer in a position to test this. On 1.4.1, a long press on the item says “scanning Gcode” (on release) just the same as a short tap.