Back up doesn't start

Start with apps/backup/lib/Cron/Backup.php, especially line 126 in runDifferentialBackup, and review how the scheduled job reaches that point. Reproduce the failure with Nextcloud 24.0.5 on Yunohost and Nextcloud 24.0.2 on Ubuntu, using external storage. Done means the backup job starts and completes without this logged error.

I have followed the documentation on how to set up back up, but it doesn't start.
I am using Nextcloud 24.0.5 via Yunohost and trying to make a back up and send it to external storage, Nextcloud 24.0.2 on Ubuntu.

This is the error from the log:
{"reqId":"RlUrBqqTpTjOQxbbrja4","level":3,"time":"2022-10-18T08:00:04+00:00","remoteAddr":"","user":"--","app":"backup","method":"","url":"--","message":"error while running differential backup - [{\"file\":\"\\/var\\/www\\/nextcloud\\/apps\\/backup\\/lib\\/Cron\\/Backup.php\",\"line\":126,\"function\":\"runDifferentialBackup\",\"class\":\"OCA\\\\Backup\\\\Cron\\\\Backup\",\"object\":{},\"type\":\"->\",\"args\":[]},{\"file\":\"\\/var\\/www\\/nextcloud\\/apps\\/backup\\/lib\\/Cron\\/Backup.php\",\"line\":115,\"function\":\"runBackup\",\"class\":\"OCA\\\\Backup\\\\Cron\\\\Backup\",\"object\":{},\"type\":\"->\",\"args\":[1666080004]},{\"file\":\"\\/var\\/www\\/nextcloud\\/apps\\/backup\\/lib\\/Cron\\/Backup.php\",\"line\":97,\"function\":\"manage\",\"class\":\"OCA\\\\Backup\\\\Cron\\\\Backup\",\"object\":{},\"type\":\"->\",\"args\":[]},{\"file\":\"\\/var\\/www\\/nextcloud\\/lib\\/private\\/BackgroundJob\\/Job.php\",\"line\":54,\"function\":\"run\",\"class\":\"OCA\\\\Backup\\\\Cron\\\\Backup\",\"object\":{},\"type\":\"->\",\"args\":[null]},{\"file\":\"\\/var\\/www\\/nextcloud\\/lib\\/private\\/BackgroundJob\\/TimedJob.php\",\"line\":60,\"function\":\"execute\",\"class\":\"OC\\\\BackgroundJob\\\\Job\",\"object\":{},\"type\":\"->\",\"args\":[{},{}]},{\"file\":\"\\/var\\/www\\/nextcloud\\/cron.php\",\"line\":151,\"function\":\"execute\",\"class\":\"OC\\\\BackgroundJob\\\\TimedJob\",\"object\":{},\"type\":\"->\",\"args\":[{},{}]}]","userAgent":"--","version":"24.0.5.1","data":{"app":"backup"},"id":"634e7dce465c6"}