Plugin s cant be enabled/disabled elgg version 2.3.5

My plugins can not be enabled or Disabled individually via the administration page.

However if i use the "enable all" or "disable all" options they work? how could this be?

my permmissions to /var/www/html/mod are recursively  // 777 www-data:www-data


Elgg version :

Release - 2.3.5, Version - 2016092300

Server : Apache/2.4.25 (Debian)

Web Server

Server : Apache/2.4.25 (Debian)
Log Location : /var/log/apache2


PHP version : 7.0.19-1
PHP ini file location : /etc/php/7.0/apache2/php.ini
PHP Log : Web server's error log
Memory available : 67,108,864
Memory used : 4,009,296
POST maximum size : 8,388,608
Upload maximum size : 5,242,880 


Memcache is not setup on this server or it has not yet been configured in Elgg config. For improved performance, it is recommended that you enable and configure memcache.