I think I found the solution to my problem with Studio One 5 constantly crashing/freezing
In my case I have 3 hardware keyboards Moog One, Rev 2 and Nord Lead 4. They are all connected to my mac, one USB input for each synth. No hub. Normally you are supposed to send and receive midi on 16 channels for each synth. Now here comes the tricky part. Studio One 5 seems to merge the midi signal from all inputs, as if they all came from one midi port. In such case you can´t have the same channel number on any synth. This was the case with my set up. I had activated the channels for each synth in the preference>external devices and by that thought it was all good. But no....this must be a bug because I can´t see this problem in Studio One 4.
So guys if you have several outboard synths or other midi devices please don´t put them on the same channel even if they are connected to different USB ports. Hope this will solve you problems...