in lib.php, on line 794 the following code -

if ($modinfo->groups === null)

{ // Load all my groups and cache it in modinfo. $modinfo->groups = groups_get_user_groups($course->id); }

is trying to set a private property of modinfo so will fail (not certain how it would actually come to happen though as the examples I have seen never permit $modinfo->groups to equal null)