Light leak from WLF on P67 long exposures ?

[Photopathe]

I am trying to figure out what hapenned on these long exposure shots (around 30 seconds). I was using the WLF, the 45mm lens (and a tripod obviouly). I think I left the WLF opened after focusing. Is this a known problem? Can the WLF cause light streaks on P67 long exposures ?
Thanks!

 

[John Wiegerink]

I'm assuming these were all shot with the MLU switch on? Take the lens off and cock the shutter and then trip the mirror lockup. Now look to see if the mirror is all the way up with no gaps. You can lightly touch the mirro in the up position to see if there is any play. Now, with the mirror still locked up, use a cable release and set the shutter to "B" bulb. When you trip the shutter release watch the mirror to make sure it stays in the total lockup position.