Nicolas: We had the same problem, but for differente environments of Groovy, you are using it through the groovy-post-build, I was using it through the SSH CLI groovysh. I solved this for my case, and will make a pull request (basicly you have to change the classloader to use the uberClassLoader. I attained it with the following diff:
- final ClassLoader cl = Thread.currentThread().getContextClassLoader();
+final ClassLoader cl = Jenkins.getInstance().getPluginManager().uberClassLoader == null ? Thread.currentThread()
+ .getContextClassLoader() : Jenkins.getInstance().getPluginManager().uberClassLoader;
You just have to do the same to the classloader on the Groovy interpreter in groovy-postbuild. If I find some extra time, I'll try to fix that plugin and make a pull request, but no promises (it should be straightforward though if you have some experience with java and git.