I've been trying to replace a world my friends have been playing on, onto a realm world so we all can play whenever we wanted to. Unfortunately, trying to upload the world results in the importing process always ending up to "An Error Occurred"
Exporting process is fine, but importing doesn't even start and always end up to that error message. I've seen people say to just be persistent, and it'll work eventually. 
The world is only 81.1MB 
In the image attached, the top bar that says "Failed" would be "Initializing Import" before the error message pops up.