Newsletter plugin not working

Hi!

After creating my blog, I tried some feed/newsletter plugins, but they all disappointed me. Finally, I installed the plugin called “Newsletter” and I was very happy to see that it worked exactly as I wished (e-mails were sent manually, I could see the list of subscribers, etc.).
Today, I updated WordPress to 3.2.1 and the “Newsletter” plugin to the version 2.5.2. Now, the newsletter option is not working anymore!
All the plugin settings seem to be ok. Under “User guide”, I found the following text:

On email list panel there is a delivery engine next run. If it is negative or empty or zero (always) the WordPress cron system is not working.

My value:
Delivery engine next run: -1313346989 seconds
… and getting worse.

How can I solve the problem? If there is nothing that can be done: how can I re-install the previous versions of WordPress and/or of the “Newsletter” plugin?

http://wordpress.org/extend/plugins/newsletter/