I got this error when calling a C# web service from javascript. With a message like that, you'd think
there was a problem with the Content-Type on my HTTP request.
Using the fine LiveHTTPHeaders
Firefox extension, I saw in the headers:
Content-Type: text/xml; charset=UTF-8
Ok, so the content-type seemed ok, and it was time to start looking at my most recent changes to see where the bug was.
After a bit of hunting, I found the problem. My web service had an int parameter, and I was passing in 't1245'. That
caused a casting error, which .NET cleverly reported back as a problem with the content-type.
Moral of the story: make your error messages meaningful.