I went through the server.log provided by you and found the following exception.
"Caused by: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: Unique index or primary key violation: "PRIMARY_KEY_C ON PUBLIC.ORGANIZATIONALENTITY(ID)"; SQL statement:
insert into OrganizationalEntity (DTYPE, id) values ('Group', ?) [23505-161]"
This exception occurs when you have some duplicate entries in your ORGANIZATIONALENTITY schema. so you can chekout your ORGANIZATIONALENTITY schema. Usualy happens with the admin user.
I had the same problem in jBPM 6.0 CR2.
Try to run ant clean.db from build.xml file.
It solved my problem.