@doboss: First, thanks for the answer.
I have done what you have suggested but without success. The error still occurs.
I have simply added the following line at the end of standalone.conf: JAVA_OPTS="$JAVA_OPTS -Djbpm.enable.multi.con=true".
Could somebody help me?
by the way: I have attached my bpmn file representing my process, what i want to model.