Hey Tihomir,
i think I figured out the problem. I can reproduce the problem with the variable assignement like this.
1. I create a sample process in the designer with my work item
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-724485-18215/450-237/screen1.png
As you can see under assignements the property of the workItem "Rechnung" is preset correctly. But when I try to map it to a process variable
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-724485-18216/450-243/screen2.png
You can see on the assignement tab Rechnung->pro_rechnung. When you then open the assignement editor again everything is empty
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-724485-18217/450-236/screen3.png
Maybe it is an encoding problem. Instead of Rechnung->pro_rechnung ist should be Rechnung->proc_rechnung
