>> I tried but the behavior is the same, when dragging the task into my process, it just pops back to palette ( but green hook is shown before!). <<
Sorry I have not seen this behaviour yet and would like to get to the bottom of it. Are you getting an JS errors? Can you show your workitem definitions configuration file so I can try to reproduce? What is the designer version used?
>> Does this mean i have to pack my work item java implementation into the .jar file that i have to upload? I really need to control my tasks by Java. <<
Yes, you have to provide the handler implementations to the runtime classpath for your runtime to be able to pick it up and use it
>> EDIT: Another really important question for my company is: when will be a release of JBPM 5.2 ? <<
It is scheduled for end-of-the-month.