Hello everyone,
Thank you for your suggestions. I tried them without success untils I realized, that I have made a stupid mistake.
In the controller sending the notification to the camunda process I had added an unnecessary @Transactional annotation, which was causing the problem, because camunda did correctly (like configured by me) open a new transaction for the service task. If that service task failed then the unnecessary transaction on hold in the controller produced the UnexpectedRollbackException.
Thank you @rob2universe
Bernhard