I have a GWT application which uses Resteasy as backend api/service, the project works properly, but when I moved the project to a different machine and run it I get this er
I don't know if your problem is the same but, I've faced the same error and find out that it was related with Firefox version > 10.0. For some reason, Firefox converts any "!" character into something like "%21".
In my cenario, my application was triggering this error while rendering a image due to this fact. The same was NOT happening on Chrome.
That situation can be handled by implementing a Filter to treat the request.
If this case matchs your problem, take a look on this post: javax.faces.FacesException: Error decode resource data while loading JSF page