Mainstage 3.2.3 crashes when using outputs 3 upwards

Some of my MainStage projects are crashing and I seem to have narrowed it down to the fact that some presets are using channels strips that have their outputs pointing to outputs on my sound card other than the main stereo 1&2. So if I use outputs 3 upwards then the MainStage simply crashes out when trying to load the project. If I setup a new project and then create new preset and select output on one of the channels strips then it just crashes MainStage. Please help!!! I have a gig very soon!

El Capitan 10.11.3

MacBook Pro Retina Dsiplay

Mainstage 3.2.3

I get the same problem with both my sound cards - Safire Pro 40 and NI Komplete Audio 6 - latest drivers used for all. I have updated all my VST plugins.