FDF can't find it's host form for some users

I have an Acrobat form hosted on the company network and users submit an FDF file by email (Outlook) to a single recipient for approval. This has worked just fine for ages, but recently we have seen an increasing number of instances where particular user's FDFs do not 'find' the form master when opened by the recipient. The name of the form is correctly shown and we can see nothing different in 'properties', but we get the error message:-

"The file you are attempting to open contains comments or form data that are supposed to be placed on . This document cannot be found."

ie, instead of looking for the correct master file name, the fdf is looking for "."(a period).
So far as I can quickly establish, all users are using the same versions of WinXp, Office2003, and Adobe Reader 8.

I know mail submissions are potentially subject to problems, and accept the risks, but as we only run in a controlled environment would appreciate any clues that may enable a fix.