Sometime, problems will be solved themselves automatically.
As one of the top 10 free WordPress plugins for database backup, BackWPup did NOT work well for WordPress 3.5 on Free Nuts.
Which means that I could NOT back up the database and send it to my email box automatically, though I could do that manually.
Without any ways to solve the problem after a lot of research, I felt upset and wanted to give up.
But luckily, WordPress has been updated to 3.5.1 later, with tens of bugs fixed, which makes BackWPup being available to back up the database and sent it to my email box automatically again now.
So, when a plugin is not workable suddenly, the reason may be on the plugin itself, on some other active plugins, or even on WordPress, and sometimes the problem will be solved itself after the next update of WordPress or one plugin.