bug in Ticket::lookup()

when i click on "closed tickets" in our system, it just displays 20 Tickets, doesn't matter which order i choose.

i just upgraded to german modified 1.9.11

after some reasearch it turned out, that line 521 of include/staff/tickets.inc.php causes this at the 21. ticket
if(($ticket=Ticket::lookup($row['ticket_id'])) && strtolower($_REQUEST['status']) != 'closed') {

in more detail, when $ticket= new Ticket($id) at the lookup-method (line 2305 in include/class.ticket.php) is called, it crashes!

i solved it temporary by change
if( strtolower($_REQUEST['status']) != 'closed' && ($ticket=Ticket::lookup($row['ticket_id']))) {

butthis is just a dirty workaround.
any hints whats the real problem?

cheers
andi