This issue has been reported here before with no solution I could find. I can confirm that my backup attempts fail at around 30% consistently. This would occur when backing up to both Nextcloud and USB flash drive. It appears that GrapheneOS has an older version of Seedvault (11?) which wasn't meant for Android 13. I am not sure if that is the underlying problem since I am not sure how to view the logs but any guidance will be very much appreciated.