Hi @cobalt ,
In general if you send a BPMN error you should get a 204, if the call is successful. If you get back a 500 that means that the process instance could not resume the call. So the BPMN error is not handled on engines side.
how are your service task implemented? Are you using connectors? Then this discussion might help you here.
In general you might want to check if you could implement your service task as external task workers, as sending a BPMN error is a little bit more straight forward and can be implemented by using this REST call.
I hope that helps, cheers
Nele