If the course_category database in moodle have a course_category which is your own parent (course_category->parent=course_category_id)
moodle do an infinite loops trying to build course_category tree, and consumig all memory in the server.

i suggest to avoid this add a line in lib/datalib.php

on the function get_categories

in the foreach (line 802):

foreach ($categories as $key => $category) {
if (!$category->visible) {
if (!$creator)

{ unset($categories[$key]); }
}
}

chage to:

foreach ($categories as $key => $category) {
if (!$category->visible) {
if (!$creator) { unset($categories[$key]); }

}
//Changed code
if ($category->id==$category->parent) unset($categories[$key]);
}