Wow, you weren't kidding! I assumed there was text along with that error.

I did a little googling and found these references. Maybe one will help:

http://www.add-ins.com/support/micro...-error-400.htm

http://social.msdn.microsoft.com/For...p?forum=isvvba

http://www.excelforum.com/excel-prog...-400-mean.html

http://excel.questionfor.info/q_micr...el_141010.html (this one has a list of a dozen or so links to similar-sounding questions)

I didn't read them all carefully (just enough to see that there were some suggestions in each about what to do), but I notice that at least two say it may have to do with an add-in that needs to be fixed, perhaps uninstalled and reinstalled. That would go along with the fact that two of us couldn't reproduce the error on our own machines.