I have been experiencing the same thing on Windows 10 edition. I've tried several times and in various ways.
Download world, play locally for a bit, attempt to replace Realm, exports successful, get the "An error has occurred".
I'd just restore from a backup but I've made some changes I really want to keep, so my options are basically to cancel the Realm until this is resolved.
I even attempted to sign up for another account by using the "Play on Realms" button from my local game. However, the Realm gets seeded with a random world, not the world I selected. Then, attempting the replace the world with the newly create Realm also fails. I'm beyond frustrated.
Now I have two realms - with worlds I don't want to play on. Go figure.
Downloaded yesterday to Kindle Fire 10 and same issue, crashes before welcome screen
My video link: https://youtu.be/4uppZZLmC_0
Summary: Using Windows 10 Edition (v1.16.210) unable to replace existing world for Realms Plus account especially if world is somewhat large (100mb+?). I attempted this with a smaller world and it seems to work (4.5mb). Also, related issues - unable to use local world's "Play on Realm" button - just errors out through the process - then the new realm is seeded with a random world. Also, seem unable to "reset" the world of the original realms plus subscription.