Debug ruby:drb (Distributed Ruby) with RubyMine (5.4)

Normal RubyMine debug configuraton for e.g. standard Rails server is working.

I would like to debug a DRB server, e.g. started with:

DRb.start_service("druby://0.0.0.0:8999", Processor.new)
DRb.thread.join

When the service is called, it returns succesfully - but not activating the breakpoint.
RubyMine even displays the execute-able line at this breakpoint, but the DRB process already completed his task.

Maybe I misunderstand the debugging concept here?

Any help very much appreciated!