Thanks for your reply.
For me, it was a jar conflict which somehow led to this problem (the message is not very intuitive if you ask me).
Anyway, after spending a considerable amount of time searching for the conflicting jars (I had about 80 jars in my application lib and some of them were conflicting with the JBoss ones), I decided to go the "hard working way": I removed all my jars and replaced them one by one when JBoss was giving a ClassNotFound exception on deploy (thank God for JBoss hot deploy, otherwise I would still be at it as we speak).
I had help from a good tool (jarscan) which can find a certain file (the class JBoss was missing) in a folder full of jars.
So, a few hours later, I managed to find the "minimum configuration" for my application to deploy successfully on JBoss, which in my case kept only 25 out of the 80 jars I was using on Tomcat.
Hope this helps someone with a similar problem.
All the best,
Razvan