GWT ErrorMessage caught : Class Not Found: rO0ABXcEAAAAAA==

Hello,
Trying to validate my test case, I have this message error "GWT ErrorMessage caught : Class Not Found: rO0ABXcEAAAAAA==".
We have already added GWT libraries and the gwt.rpc file.It seems that the response could not be decoded properly. I don't understand what is missing.
Many thanks in advance.