Patrick,
Here's a clue from my intranet W2k server. If my form contains 4M of attachments I hit the httpruntime maxrequestlength default limit of 4096k, I get my error message, as I should. If I increase the maxrequestlength to 8192, it works again. Seems to me my original difficulty might be an IIS running on XP issue. Here's hoping...
Clive