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

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

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.