@eva

You should be able to use the Syphon output from stage setup to allow OBS to grab the input. 
Using OBS is a good choice for big video like this, it allows completely separate cpu/gpu processes to deal with the video recording, which will ensure Isadora provides the best possible playback
Yes, set OBS to record at the speed you set the Isadora framerate at.
Since you are on Mac you could try Syphon recorder, it may be the easiest option. Syphon Recorder | Syphon

Ok, I need sleep, Im going by train tomorrow to the festival where I need to test the work tomorrow night. Help is much appreciated, thanks !!!!

I changed the framerate today - can that make a difference? Its the only thing I did different from earlier

@dusx

I just retried with apple pro res and quicktime. Its all black

@dusx

I changed the framerate - Is that the same framerate that will show in OBS? If I can possibly record there in stead?

oh sorry, I did do that. But it doesn't work. I set it to Apple pro res 422.

I need to go to bed. I will check tomorrow in the morning before I leave. 

Please try setting your Output/Record Stage Settings, to use container 'quicktime movie' and codec 'Apple Pro res 422'

It is likely that the mp4/h264 codec was not allowing the ultra wide format of your blended stage. 

@eva said:

like change the frame rate

 You can set the projects target framerate in the preferences. Open menu, Isadora / Preferences -> General Tab, Frame Rate/Tasks

here is a zipped filewalking-towards-peace-essen2-mapping-kopi.izz.zip