Fatal error: Using $this when not in object context

Hello,

I just wanted to set up the last versions of Tooltip editor and Elgg 1.8-1.12 & 2.X: Extended TinyMCE plugins. After downloading plugin folders, I deactivated CKEditor, and now the whole website is crashed (can't remember if I activated new plugins before crash). I now got this error: Fatal error: Using $this when not in object context in /home/xxxxxxx/xxxxxxxxx/html/vendor/elgg/elgg/engine/classes/ElggSession.php on line 393 

The line in ElggSession.php is this one: if ($this->has($key)) {

I read it could be a PHP version issue, but there is a 5.6 version on my shared server. ^^

Any idea on how to solve this?

Thanks in advance!