Newbie: MPC X - PlugIn-track - sound only out of headphones

I just created a track with a internel Plug In, and can hear the notes I'm playing on the pads out of the headphone.

But there is no sound at all out of the 6,3 mm main outputs on the back, although the "Audition-outputs" in "Preferences" are set to " Out 1, 2".

What could be the reason?

Go to Channel Mixer > Programs > Route. What output pair is your plugin routed to? It should be 1/2 by default, but it sounds like it might be going elsewhere.

You can check:
1. Is the output volume knob not at zero position?
2. is the input monitoring knob not set in max left position?
3. is the cable from main out to the speaker working 100%?
4. are speakers working with other sources?

Thanks for your help!
Got it solved now, it was infact a unexpected cable problem.