Dialog sticks on the left side in UI 1.8.5

Dialog sticks on the left side in UI 1.8.5

Everything I try to center the dialog horizontally fails: 

position: 'center'
OR
position: ['center','center']
OR
position:[300,'center']
OR
$['#my_dialog'].dialog( 'option', 'position', [320, 500] );

Only thy vertical positioning seems to work. This happens with the 1.8.5 UI while 1.8 still works fine. Unfortunately I need some goodies from the new UI version... Could someone tell me how to center the dialog? Is there "a new right way" to do it or is it just a bug? 

Adrian