I had this problem sometime when I rewired my little studio. For some reason Windows does not like it when you plug a USB midi instrument into a different port than it was previously in. I have a couple of KORG instruments which comes with a nice utility to uninstall the MIDI drivers and therefore reuse the ports.
I understand you can use this even if you don't have any KORG gear, just download the KORG USB drivers and there is a utility called KORG MIDI Driver Uninstall Utility, just deselect the option to configure KORG gear only.
 

Anderton
Check out this article about the MIDI Port problem, which persisted in Windows 7 but was fixed in Windows 10. Don't know if it's at the root of your issue but it's easy enough to find out.

I did not have any hidden or unused devices, but thank you for the suggestion in your article.  I am running Windows 10.  From using the remote control program V-Control Pro in the past, I do have ten entries for Bome's Virtual MIDI Port Device".  The remaining eight entries are for other devices I do use.