Hello,
I’ve tried to do as suggested and after disabling all plugins followed with the same problem.
While the theme I use the widget plugin worked Events Calendar above too change the theme to Twenty Twelve and the work.
I tested reinstalling the previous version of the plugin and it works with my theme, but when you upgrade to version 3.4.1 stops working, so I think it’s due to the upgrade plugin.
So it’s possible that the error is in the widget or plugin version 3.4.1.