While I was backing up realms or applying resource or behavior packs, it fails while I tried to install it and lets no one join my realm and I repeated until it's fixed.
List of steps we've done with this bug:
1. Prepares to backup this realm.
2. Loading a backup.
3. Error message on applying resource packs. (attachment)
4. Waiting on backups to finish.
5. Failed the backup.
6. Repeat until the backup is complete.
Also, the resource and behavior packs cannot be added or removed until this is fixed.