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.
After an update of WordPress my UpdraftPlus Backup/Restore page remained blank.
Blank pages in the admin area usually mean that your site is running very close to its PHP memory limit, and sometimes tipping over. To raise your memory limit, see: http://updraftplus.com/faqs/deal-fatal-error-allowed-memory-size-errors/ (I'd raise it, whether you see the text error or not - you won't usually see the text itself unless you turn debugging on and your debugging messages are set to go to the browser).
Best wishes,
David