This is a very common error message. To solve this, first you need to enable loggin in WCF. (Search for WCF Logging on Google) . Make you in logging section you enable all the types of the error messages
Eg
logMessagesAtServiceLevel="false" logMessagesAtTransportLevel="false"
maxSizeOfMessageToLog="2147483647" />
then you can see the exception on the error log file..
I got this error message because i have not set max request length, you need to add this to the web.config file