HTTP/1.1 415 Cannot process the message because the content type 'application/json; charset=utf-8' was not the expected type 'text/xml; charset=utf-8'.

Error
HTTP/1.1 415 Cannot process the message because the content type 'application/json; charset=utf-8' was not the expected type 'text/xml; charset=utf-8'.
I'm pretty sure this is a config error...but i just can't see it.
Thanks in advance.
Inspecting the http request from fiddler shows 
POST /ah3/Services/authentication.svc/Login HTTP/1.1 Accept: */* Accept-Language: en-gb Referer: http://ipv4.fiddler/ah3/Login.aspx?ReturnUrl=%2fah3%2fDefault.aspx Content-Type: application/json; charset=utf-8 Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; SLCC1; .NET CLR 2.0.50727; .NET CLR 3.5.30729; InfoPath.2; .NET CLR 3.0.30729; OfficeLiveConnector.1.4; OfficeLivePatch.1.3) Host: 127.0.0.1 Content-Length: 71 Connection: Keep-Alive Pragma: no-cache {"userName":"Peter","password":"Peter#1", ...