Got an absolute nightmare of a problem here, hours way from hundreds of
people using these forms. Any help massively appreciated.

Ive got a form published in SharePoint with open access. However, we cant
let users see the forms submissions of others, so even though the form is
hosted in sharepoint the submission is pushed to a web service, then a
privileged account pushes the form back into sharepoint into another document
library with restricted permissions. Security needs to be good so just
removing views etc wouldnt be secure enough: hence the web service.
Sharepoint is used twice so the opening and saving domain is the same and the
form posts back without security warnings €“ even though the original
open-access forms library actually stands empty.

Submission from InfoPath is failing. I get

The SOAP response indicates that an error occurred on the server:
Server was unable to process request. -- The root element is missing.

Ive enabled tracing on the server and notice that these requests result in
a 500 response at the web service. The content length of the request,
according to trace.axd is 0. Thats very important.

I installed Fiddler to act as a proxy on the client and watch the data leave
InfoPath. When fiddler is running, the submission ALWAYS works. Checking
the trade.adx file, the content length is non-zero, and changes depending on
form content.

I believe what is happening here is that InfoPath is incorrectly setting the
content length, but fiddler, after inspecting the message, adds its bits and
pieces to the header and recalculates a non-zero content length, which then
works a treat.

Any suggestions?

SORRY that was obviously about InfoPath - had word open in the background and
clicked communities on the help menu. Oh dear!

Case closed (or rather transferred)