I had a similar exception:
java.lang.SecurityException: class "org.hamcrest.Matchers"'s signer information does not match signer information of other classes in the same package
The root problem was that I included the Hamcrest library twice. Once using Maven pom file. And I also added the JUnit 4 library (which also contains a Hamcrest library) to the project's build path. I simply had to remove JUnit from the build path and everything was fine.