I’m receiving the same warning message while backing up WordPress 3.5.1 to Dropbox.com with BackWPup version 3.0.8. Only started happening after upgrading.
PHP Version 5.3.24
MYSQL Version 5.1.68-cll
cURL version 7.24.0
[29-Apr-2013 21:06:12] Uploading to Dropbox …
[29-Apr-2013 21:06:28] WARNING: is_file() expects parameter 1 to be string, resource given