AJAX file is blocked in Wordpress on Ubuntu 14.04

Having trouble with AJAX registration form on our Wordpress site after moving to DO hosting. Contacted theme developer and this is what he said:

*Please contact your host, for some reason AJAX file is blocked, it’s required for all AJAX actions in WordPress and should be accessible.

http://worldbeat101.com/wp-admin/admin-ajax.php Failed to load resource: the server responded with a status of 500 (Internal Server Error)*

Error Logs:

[Mon May 11 06:52:29.569063 2015] [mpm_prefork:notice] [pid 25013] AH00163: Apache/2.4.7 (Ubuntu) PHP/5.5.9-1ubuntu4 OpenSSL/1.0.1f configured – resuming normal operations [Mon May 11 06:52:29.569132 2015] [core:notice] [pid 25013] AH00094: Command line: ‘/usr/sbin/apache2’ [Sun May 17 02:11:45.298078 2015] [mpm_prefork:notice] [pid 25013] AH00171: Graceful restart requested, doing restart [Sun May 17 02:11:45.393467 2015] [mpm_prefork:notice] [pid 25013] AH00163: Apache/2.4.7 (Ubuntu) PHP/5.5.9-1ubuntu4 OpenSSL/1.0.1f configured – resuming normal operations [Sun May 17 02:11:45.393496 2015] [core:notice] [pid 25013] AH00094: Command line: ‘/usr/sbin/apache2’ [Sun May 17 02:17:07.218365 2015] [mpm_prefork:notice] [pid 25013] AH00169: caught SIGTERM, shutting down [Sun May 17 02:17:08.277797 2015] [mpm_prefork:notice] [pid 28293] AH00163: Apache/2.4.7 (Ubuntu) PHP/5.5.9-1ubuntu4.9 OpenSSL/1.0.1f configured – resuming normal operations [Sun May 17 02:17:08.277908 2015] [core:notice] [pid 28293] AH00094: Command line: ‘/usr/sbin/apache2’