neripolvani,
Google Plus uses the Google API client therefore needs to use curl. Curl is configured at the server level, instructions to activate curl for you or your hosting company can be found at http://www.php.net/manual/en/curl.requirements.php.
In the mean time you are going to have to disable the plugin. One way that might work with your website inaccessible is to change the permissions on the folder /wp-content/plugins/google-plus-button to something like 600.
On Linux via SSH: Change into the /wp-content/plugins folder and chmod 600 google-plus-button
Thats totally weird!! I have never seen it take down a site like that. I will be sure to update the readme and description so people understand whats required. Sorry!!!
