I had a similar issue a few months back when I updated Windows.
If it is the same problem you are facing, it has to do with Windows not recogizing more than 10 midi devices (that's why it is not affecting everyone).
Open regedit (run and type regedit)
Open the HKEY_LOCAL_MACHINE
Then Software > Microsoft > Windows NT > Current Version > Drivers32
Delete the midixx keys (see attached pic). There is probably a way to find out which are used and which are unused, but I just deleted them all.
Reboot, then you will have to reinstall the drivers for the midi devices you want to use and reboot again.
Hope it helps, it worked for me. I does involve editing the Registry, so use caution.