Thank you for your response, but, unfortunately, it wasn't this error....
I found the problem, it was due to the size of the array editorSubmissions in the functions getEditorSubmissionsUnassigned, getEditorSubmissionsInReview, getEditorSubmissionsInEditing, getEditorSubmissionsArchives (editorSubmissionDAO.inc.php)
Indeed, it cost too much memory to the system (size of an instance of and object submission * number of articles)
So I solved the problem this way :
In the array editorSubmissions, I just create a instance of submission only when it's needed : for example : in the page submissionsUnassigned, it displays only 25 articles per pages : we don't need to get all the informations of the 975 other articles.