I think that we might have discovered a problem in the area of calling a WCF Service when passing certain types of parameters by value in and out of the interface. The error message that we are receiving is similar to the one you are receiving.
Can you confirm what your service methods are using as parameters? The problem that we found occurs when trying to pass a data only object by value as a parameter in order to pass a COBOL group item. If we change this to set a byte array (type Byte occurs any) to the group item on the client side and then pass the byte array instead then it works ok.
If you would like to create a support incident with customer care and put it to my attention, I would be glad to take a look at your scenario to see if it is indeed the same problem?
Thanks.