Subscription form not working – nothing happens

Hello, first of all thank you for this amazing plugin. It is great to have this easy functionality to create and send great newsletters very easily.
To my problem: On the site i’m building for my client, the subscription form does not work anymore, i’m using it on the right as widget. This used to work, but i don’t know since when, now nothing happens when you enter an email address and click on the button. Probably it is conflicting with another plugin (ajax not working?) but i didn’t want to deactivate all of the plugins because doing that once before had removed all my widgets.

Please please help to find out what is happening.

thanks in advance!
M.

http://wordpress.org/extend/plugins/wysija-newsletters/