You need to be a root user to do that. It seems you do not have the necessary skills to handle this fix, I suggest to hire a professional to not ruin more.
Can you use this:
16-07-09 16:29:48 162.158.209.54 AH01079: failed to make connection to backend: httpd-UDS, referer:
http://www.domain.com/_ac79f3eb1.html Apache error
2016-07-09 16:51:42 141.101.104.103 AH01071: Got error 'PHP message: PHP Notice: Undefined index: register-form in /var/www/vhosts/
www.domain.com /httpdocs/register.php on line 18\n' Apache error
2016-07-09 18:16:00 31.223.136.220 AH01071: Got error 'PHP message: PHP Notice: Undefined index: register-form in /var/www/vhosts/
www.domain.com /httpdocs/register.php on line 18\n', referer:
http://www.domain.com/browse-date.html Apache error
2016-07-09 20:38:05 188.2.97.245 AH01071: Got error 'PHP message: PHP Notice: Undefined index: register-form in /var/www/vhosts/
www.domain.com /httpdocs/register.php on line 18\n', referer:
http://www.domain.com/_4c363b7a9.html Apache error
2016-07-09 20:38:34 (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /var/www/vhosts/system/
www.domain.com/php-fpm.sock (*) failed Apache error
2016-07-09 20:38:34 141.101.96.61 AH01079: failed to make connection to backend: httpd-UDS, referer:
http://www.domain.com/59db30075.html Apache error
2016-07-09 20:38:34 (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /var/www/vhosts/system/
www.domain.com/php-fpm.sock (*) failed Apache error
2016-07-09 20:38:34 141.101.96.61 AH01079: failed to make connection to backend: httpd-UDS, referer:
http://www.domain.com/59db30075.html