It's either this PHP bug:
https://bugs.php.net/bug.php?id=55169
Or it's a problem with the setup of your web hosting company:
http://wordpress.org/support/topic/drop-box-authentication-error?replies=2
a) Which versions of Windows and PHP is your web hosting server using? (If you don't know, then click on 'backup now', then after the backup starts, click on the link to download the most recent backup log, and copy-and-paste the top 5 lines here).
b) Regardless of a), try this version anyway:
http://downloads.wordpress.org/plugin/updraftplus.zip
Then please let us know if that works.