When I click the 'Login to see the rest' link in post following error is displayed,
Forbidden
You don't have permission to access /wp-login.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
How can I fix this?
http://wordpress.org/extend/plugins/are-paypal/
It looks like the plugin fails to find wp-login.php url. It can be that environment variable "blog url" is corrupted or wp-login.php is dropped or disabled by some other plugin.
Goto http://arepaypal.ehibou.com and comment on one of the posts about your problem. I'll contact you.