Hi,
@Post 1
If the solutions from post 2 didn't work, Try these as well:
If Reaper is throwing out an opening device failier, It's iether for a lost connection between Reaper and the midi device or probably nonexistant or damaged device driver.
Double-check the cable connection between the midi device and the computer and make sure you have your midi device plugged in correctly.
If your problem is due to a software misconfiguration or something like nonexistant device driver, Reinstalling the device driver may help as well.
Also if you've unplugged your midi device while Reaper was open, you need to exit and reopen reaper for the device to be reenabled. (Just how post 2 pointed out)
@Post 2
You can find "Forget device" by right clicking or pressing the applications key on the selected midi device in the midi devices preferences.
Co-founder of Sonorous Arts.
Check out Sonorous Arts on GitHub:
https://github.com/sonorous-arts/