The infamous CKErrorDomain 15

That is a server error code. It is not a sync error. As we don’t control the server, there’s nothing we can do about it.

Evernote does not use CloudKit.

We are not the only developers who have had issues with CloudKit.

The problem is sporadic and doesn’t affect everyone. Why? We have no common vector at this point.

Apple’s device backups are not using the same mechanism.

Apps syncing just via iCloud Drive are not using the same mechanism either.

Sync data is not a copy of your documents or database(s) so it doesn’t truly matter what Dropbox has technically done. This is even more true when using an encryption key. If you choose to be morally or philosophically outraged, that’s an unrelated matter.