There seems to be a problem in classes/Category.php file.
The CurUserCanAddFiles() is called with no $user parameter.
Original code is:
function CurUserCanAddFiles($user = null)
{
return $this->CurUserIsOwner($user) || ($user !== null && user_can($user, 'manage_options')) ;
}
My fixed version is:
function CurUserCanAddFiles($user = null)
{
if(is_null($user)) $user = wp_get_current_user();
return $this->CurUserIsOwner($user) || user_can($user, 'manage_options');
}