Hi,
Please can you provide a link to your log file for the problematic backup? (You can download it from the "Existing Backups" tab of your UpdraftPlus settings page).
It'll be too long to paste into the forum here, but you can download it to your computer, and share it with Dropbox, or paste it in pastebin.com, or any similar service, and post the link here.
Best wishes,
David