There is currently a problem with the MIDI output ports in Notion and Windows 10. The issue has been logged with the development team in hopes that it might be resolved in a future update. In the meantime, please go to File>Preferences>MIDI and select "-" for all of your MIDI output ports. Once you have set your MIDI output ports to none, please close and re-open the program. If the ports are already set to "-," please set them to something else, and then back to the hash-mark. Close and re-open Notion.