Bad request with invalid data at root level.

Question

This error has been haunting me for days now. Have enabled all sorts of logging, and making all kinds of tests, without luck. Other clients connects and sends quite large (5-30Mb) messages without any problems. This single client works ok up to 5Mb messages, but larger messages creates a bad request response at the server. The exception logged says that there is "invalid data at root level, line 1, position 1. " The client failing is configured exactly as the other working clients.

The binding is wsHttpBinding with Message security. There is properly set large values for sendTmeOut and ReceiveTimeout, and large content length parameteres, bt i assume this is not where the problem lies, as there is no problem from other clients. Exercises with setting these values "wrong" has all given other reasonable exceptions in server log. 

Seems as if something is corrupting the http request from this single client, and i have no idea what that may be...

Suggestions may be at great help.