I'm having trouble using Word 2003 Mail Merge with (.csv) and (.txt) files.
Works fine with Word 2000. Problem starts with message: "word cannot find
its data source". You can "remove data/header source" or "remove all merge
info" and then reconnect the data file. Everything works fine until you save
the Word document and then try to open it again. It has the same problem
with the data source file.

This and/or similar problems with the mail merge have been reported for at
least two years, but I cannot find any reference to a fix. Things work fine
if you use an Excel data file, but at the current time that is not an option
for this multi-user application.