I find it strange that the datalogger contains the data that for some reason was not sent to WD.
Another thing to try is a different PC in case there's something about the serial port or its driver.
You could try the 2wd.txt method to run another instance of WD. A great troubleshooting technique that isn’t well documented or understood, the FAQ explains how to use it.
Look for “Multiple instances of WD on same computer” (http://discourse.weather-watch.com/t/10889)
Another possibilty is to use the Davis Weatherlink software for 24 hours to see if that captures any "out of range" values. [Edit]Now running Weatherlink to see if it lists any strange values[\Edit]
A great idea. Hopefully provides some clues to solve this mystery.