Can't backup after upgrading Drupal core 3rd May 2012

I upgraded to Drupal 7.14 yesterday (from, I think, 7.12). I used the Backup module (which is brilliant) to back up the database to one of my own computers before upgrading. Upgrade went fine. This morning I tried to back up the database as I do every morning, and I got the very unhelpful message "Could not complete the backup."

I have changed no settings since yesterday, when the backup worked as it always did. This morning I took the site into maintenance mode and cleared all the caches, but it made no difference. I can access the database through phpMyAdmin, and the site is functioning fine, so there wouldn't appear to be a problem with the database itself.