What all happens EXACTLY if I clear the cache? What will get deleted?
When compiling your project, GMS2 will store some of the work it did in the cache.
That way, next time you compile, it doesn't have as much work to do and can get the data it needs from the cache instead of compiling everything from scratch.
However, sometimes, you will change something in code, but rather than compiling the change, GMS will used the cache version (who knows why? Hash collisions maybe?).
Clearing the cache will make sure that GMs compiles everything from scratch the next time you hit run.
However, it just sounds to me like the editor thinks the enum still exists.
In which case restarting GMS should solve the issue.