The reason why I run into this problem is that I moved the site to a different server, and the logs, backups and temp paths of BackWPUp are absolute – consequently they have to be changed. But there is no way to change the temp path. In this case the solution is easy:
– delete and reinstall the plugin (it may not be necessary, but who knows…)
– browse the table wp_options in the database and change _all_ the references to the absolute path on the old server to the absolute path on the new server
Why is it so, who knows…