Mission editor stops loading missions after using it for some time, for example loading different missions to edit them may cause ME to just stop loading anything and needs DCS restart. It may even continue doing that after restarting the DCS. Also it can freeze so that if you press button, the button stays highlighted and nothing happens, press another button and it gets highlighted too and so on. Only solution is to restart DCS. Also another general issue, many times after quitting mission, screen just turn black and nothing happens, solution? Restart DCS. This has become a new minigame in DCS.
These look like similar issues in DCS 2:
https://forums.eagle.ru/showthread.php?t=177900