The permissions check is based on the assigner user access level instead of the current user. This means the current user have rights to assign issues to someone but this someone has no the same access level, the action is refused. On the contrary the same action is accepted if made using the single bug view.
The following line:
access_has_bug_level( $t_threshold , $t_bug_id, $f_assign )
should be modified in:
access_has_bug_level( $t_threshold , $t_bug_id )
where the permissions check is based on the current user. | 
