People seem to be reporting problems in this area more and more. Perhaps
that's because DDE has been an obsolescent technology for 10 years, and/or
because MS makes more and more security-related changes to Office and
Windows, or maybe it is just an illusion.

If you see the message from time to time, or quite a lot, but not always,
then I would suggest there could be a timing problem or a problem with more
than one application trying to use the same Excel file. e.g. perhaps you are
opening one mailmerge main document while another that uses the same
workbook is still closing.

if you see it all the time with particular workbooks, the workbooks may be
corrupt and if you are able to export (e.g. in CSV format) and re-import, it
may make a difference.

Otherwise,... it could be anything, really. Could be worth renaming &
delating Word's Normal.dot, saving and deleting Word's data key in the
registry, and so on (there are relevant articles at word.mvps.org if you
need further info on that sort of stuff). Could be that something is leaving
your workbooks "read-only" (seems unlikely). Do you have any further clues?

Peter Jamieson