Hi, it seems that the issue is closed, but I have been on the same case as Edlimar.
I have moved from 4.2.2 -> 4.2.6 -> 4.2.8 , and suddenly most of the JPA based functionality was failing. I have crossed check again if i load (or through another dependency) the java-assist jar. Currently in my effective pom, it is only the hibernate core or entity manager that 'bring it along.

I am using Websphere 8.5.5 sp1 ,I am not sure if there an internal conflict between classloaders, but IF java assist 3.18.1.Final was the case then the application would be fine since it is already loaded. What I am trying to say is that javassist 3.18.1 is loaded both on 4.2.7 and 4.2.8. Hibernate works smoothly on 4.2.7 while on 4.2.8 breaks.

It would be nice if you could further investigate (re-open the issue)