Why wouldn’t you have told us what that line added to the wp-config was? I am having an issue where I can’t edit files in visual mode. If I go in there and hit enter nothing appears to happen but if I switch to text mode you can see the  ’s in there. Really strange.
It appears there are tons of variations of “broken visual editor” issues out there, all with different causes, so tracking down a fix is very tough.
It would be nice to know what line you added to the config to fix yours so I can try it?
Thanks.