I upgraded to 6.6 a few days ago and everything was fine. Then I packed up my laptop and went to band practice on Thursday night and my MIDI controller wouldn't work with some songs, but was fine with others. I worked out that it was only working with older songs, which I had not saved since the upgrade. I had a second controller there so it set it up and it wouldn't work, either. The next morning, after I set my laptop up at home again, the controller I have at home wasn't working with any of our songs.

So that was three different controllers, from three different vendors - Korg, Arturia and Donner - that wouldn't work with Studio One. However, they all worked fine with half-a-dozen standalone synths I have installed and with my old DAW, Orion. I checked the settings and whichever controller is plugged in seems to be recognised by Studio One - no little yellow triangle - but it still says "not connected". I tried restarting Studio One and restarting my computer and all the rest of it, to no avail. Eventually, however, I solved the problem...

SOLUTION: Remove the controller from the list and add it back in. It's a relatively pain-free fix but I left it until last. You should try it first if you have some the same problem.