Well, I have pretty much answered my own question.
After doing some searching on GitHub, I came across Issue #303 ( Allow POST of data, even when one variable UUID is wrong), which got me looking hard at the .csv data I was trying to upload. It really didn’t become clear to me until I grabbed some new data from the sensor last night.
If you have four headers defined (time, water temp, voltage, board temp) and you drop the last one so there are only three data fields, the upload will fail. Just one of the pitfalls of touching the data too much.
Funny in a way because I had changed the code in my logger (some time ago) to output the exact format I need to upload but haven’t been out to the sensor to upload the new code.