OJS blank page after new user registration

i use OJS version
after new user filling registration form and click Registration Button, there were not appear anything. only a blank page appear.
whereas the cache folder is already writable recursively, temp folder is already empty, and I have tried to restart apache and mysql. but still can not solve the problem.
please help how to fix them.
please could try registering at the link below

A blank page almost always indicates a PHP error which was captured in your webserver’s error log. Locate that error and we’ll have some more to go on.

In depth troubleshooting is described here:

I have opened error.log, the error message that appears is:
"PHP Fatal error: Call to undefined function curl_init () in /var/www/plugins/generic/stopForumSpam/StopForumSpamPlugin.inc.php on line 119, referer: Register | IBDA` : Jurnal Kajian Islam dan Budaya "

error message below appears:
script ‘/var/www/proxy.php’ not found or Unable to stat, referer: RefererString

The StopForumSpam plugin is trying to fetch external web content via cURL (for spam checking), but cURL is not installed or enabled on your system.

The PHP documentation can give you more information on cURL:

Another option would be to disable the StopForumSpam plugin. (But you probably want cURL.)