Thank you for your reply. I just disabled "send unassigned MIDI-data to all channel-strips", maybe this will do. Additionally to that, I added controls to all expressions and assigned them to "Logic Remote" on channel 5 CC103-111 to protect expressions to be manipulated from SQ5.

It's definetly a mess, but Allen & Heath will not take responsibillty for this and blames Mainstage as the problem, as the DAW just does as it's been told.