HI,
sorry for the late answer.
To be honest, I am not entirely sure what solved the problem. I did a few things. One thing was to upgrade to the latest Camunda version.
But I guess it was a somehow faulty workflow definition being deployed in the workflow engine, where the classname attribute of a task was set without a value.
While writing these lines, it does not occur logical to me that such an error could have been the reason for the problem.
Nevertheless - I would recommend to strip down the setup to as few as many process definitions and process instances if you have the chance.
I am sorry for not being a good help!
Chris