I was focusing on this issue being on the server side, but from my further research it seems to be the client side.

Try this first: http://support.microsoft.com/kb/835215

Alternatively you could just search for the form FRMCACHE.DAT file and delete it manually.

If the client still receives the error, create a whole new profile for their email (yeah I know, this sucks) per: http://support.microsoft.com/kb/312354

This should rebuild their local cache which seems to be the issue.

If the above does not resolve the error, or you have any problems / questions about any of the above, please let me know.