I have the same problem. Yamaha MO8 worked fine via usb MIDI since 2010 (iPad 1st gen, iPad 3rd gen, iPad Pro 9.7 and now iPad Pro 12.9 3rd gen) but stopped working since I updated to iPadOS 15.4.1 a few days ago . Music apps like AUM, Korg Gadget… crash when connecting my Yamaha keyboard.