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.
Seedvault fails to backup
GrapheneOS uses very old version of SeedVault and is unlikely to get updated any time soon (see https://github.com/GrapheneOS/os-issue-tracker/issues/1924).
So yeah, backups are in a rough shape right now unfortunately.
nazar-pc It should be updated on the next release:
https://github.com/GrapheneOS/grapheneos.org/commit/536c76f3a00b18f1c0f8d5eb9c29590e0c8bdfc7
matchboxbananasynergy I was wondering why the issue was "closed" twice.
Very nice, hope new release works better!