Originally Posted by
dabac
1) does the frame has a fixed derailer hanger, or is the hanger clamped under the acorn nut of the q/r?
+1. A claw-mounted derailleur was my first guess, too.
If your rear derailleur is mounted via a "claw", as opposed to a hanger that's part of the frame, the claw itself and the nut that helps hold it in place take up a portion of the dropout slot on the drive side. The axle will be able to slide all the way back on the non-drive side, but not on the drive side because of the claw, so the wheel will sit crooked if you pull the axle all the way back on both sides.
The solutions are to either not slide the wheel all the way back on the non-drive side, or to put a spacer in the non-drive side dropout that takes up the same amount of space as the claw.