Debugging App Engine with --backends

I can debug my application in Eclipse without running a开发者_JS百科ny backends just fine. Now I want to debug my application with the backends enabled. I start the dev_appserver via Eclipse with the --backends. But the execution doesn't stop at the breakpoints I set. I tried setting the application ports to the same as without backends by passing --multiprocess_min_port=8080, but that didn't help either.

What am I missing?