With version 1.4.3 on my Windows server (as per previous conversations here http://wordpress.org/support/topic/plugin-wordpress-backup-to-dropbox-issues-with-v121?replies=12), the plugin stopped working, without any clear reason - it just kept saying 'process has gone away - attempting to resume). I didn't know what to do so thankfully I had only upgraded WPB2D on my site and not my clients so decided to wait for an update since I didn't have time to go through finding out what was wrong again.
I've just updgraded to v1.4.4 and seems we now thankfully have a bit more info:
It gets started fine, but then seems to fall over on the very first subfolder it finds, complaining for example:
12:12:22: Error uploading 'D:\[path]\wp-admin\admin-post.php' to Dropbox: Invalid path '/MySite\wp-admin': character at index 14: backslash not allowed
It actually first found a non-WordPress folder I had in my site, and since I didn't need it that folder I deleted it, then the plugin just fell over on the next folder it found.
Eventually it just says:
12:12:22: A fatal error occured: The backup is having trouble uploading files to Dropbox, it has failed 10 times and is aborting the backup.
Hope this will help you fix it again.