I think I found out the reasons:
They are working on updating the core components of the framework, in fact Karaf with OSGi. That means even I didn’t changed my code the build environment puts in dependencies on newer component versions than 3.0/3.1M1…3 have. Let me try to work around. Switching back to an older build environment should fix that issue, but I need to think of a proper way to keep backward compatibility. Having 3 versions (2.5.x, 3.0 and 3.1 latest) is no option.