Currently some exceptions are "swalllowed" and errors are harder to diagnose.
Example here: https://groups.google.com/forum/embed/?place=forum%2Fcamunda-bpm-users&showsearch=true&showpopout=true#!topic/camunda-bpm-users/46nDtlvVdqo

AT:

• all engine-rest-specific Exceptions are converted by an exception handler
• response has a json body of form {"type" : "NullPointerException", "message" : "some exception message"}

  (see ExceptionDto and ExceptionHandler)