If the Editor isn’t working, you likely have some sort of Javascript error which is preventing the Editor from loading propertly. This is usually causes by another plugin.
The simple way to verify this is the problem is to temporaily disable all other plugins except Email Users and see if the problem persists. If the problem goes away, you need to enable other plugins until it stops working to narrow down the culprit.
If the problem still persists after disabling all other plugins, it could be the theme. Does the theme have a custom Dashboard? If so, then there is a good chance that is the source of the problem but it is hard to know for sure unless you’re willing to temporarily change themes to see if the problem resolves.
