Ok, so we were finally able to reproduce the problem!

A private observer method which is NOT intercepted but is declared on class which contains at least on intercepted method (thus requires subclass proxy to be generated) fails with IllegalAccessError.

A workaround (except for changing the modifier) might be to intercept the observer method with some dummy interceptor.

Now we will try to identify the root cause. Stéphane Appercel Thanks for your patience and help!