[polvanacoleyen]

Update: Still no progress. In fact, I've tested Bandhelper, which also didn't work (that didn't crash at least, there were just no midi messages being sent or received). But as a last resort, I've tested an old USB-MIDI interface. That works! But only on Bandhelper. If I use that interface with MSPro, it still shows the same effect: it freezes the app.