Hi,
I solved the problem by removing the jta*** tag. The exception was removed when I used,
<property name="hibernate.transaction.manager_lookup_class">
org.hibernate.transaction.WeblogicTransactionManagerLookup
</property>
instead of
<property name="hibernate.transaction.factory_class">
org.hibernate.transaction.CMTTransactionFactory
</property>
in jbpm.hibernate***.xml.
Thanks alot for the help. :-)