On that machine, it gets the "Send error to Microsoft" popup. Not
much help other than a "Save Failed." message. Excel closes after
clicking Okay on the popup.

On the other machines, it behaves as though the save worked, and the
new file exists. But if I close Excel and attempt to open the new
file, it's corrupted. I had done a lot of work thinking it was
saving, until I couldn't open ANY of my saved files. I had to go back
to the original file from six hours earlier.

Dan