The problem has been sorted by myself! I'm using lomboz plugin to deploy the war file. To fix it, I added server/default/lib/bcel.jar to the classpath and everything worked fine.
I don't know why I have to add so many jars to my classpath in order to get lomboz working with jboss 3.2.1. That's not the case with 3.0.X where only 3-4 jars are required in the classpath.
could you specify what you changed standardjbosscmp-jdbc and standard-jaws?
did you set mySQL to default?