Hi Frank,
If a manual backup is working then your issue is likely due to low visitor numbers at the time of the backup.
UpdraftPlus can only make a backup when WordPress is loaded into Memory. This normally happens when someone visits the site.
I would recommend setting up a number of dummy visits to the site via an external cronjob.
These are automated tasks performed by the server.
You will need to set up an automated task to visit your site (please set the cron job to visit: http://yourdomain.com/wp-cron.php?doing_wp_cron )every 5 – 10 minutes (This will also keep the rest of your WordPress scheduler running):
Your hosts may have a tool to set up a cronjob, or you can use an online tool such as EasyCron