I visited your blog and viewed the error. Your host server is definitely running in "PHP safe mode". This "PHP safe mode" restricts access to only the directories specified in your ISP's setting, in your case (/home/vivi/, /tmp, and /usr/local/lib/php/). A support tech at your ISP needs to see this error because it looks like there may be a server misconfiguration problem where the allowed paths does not match the real path of your own site. If you can, copy and paste the error in an email and send it to your ISP support email address. That may be the only way to guarantee it will be seen by someone who understands server configuration.
Because of this problem, Wassup is unable to access the default path for PHP session_save_path. Your getting a blank screen even after deactivating Wassup confirms that this problem goes beyond Wassup and is on your host itself.
I am not optimistic that this will help, but you could try entering a different "save path" value in Wassup Options. Go to the Wassup==>Options or type the URL "http://yoursiteurl/blog/wp-admin/admin.php?page=wassup-options" in your browser. Then go to the "Manage Files&Database" tab, and enter a new value for Temporary files location folder. If the value is "/tmp", you need to change it to something like "/usr/home/vivi/tmp". If the value is "/var/tmp", change it to "/tmp", instead, then save. Good luck.