Yes! at last someone else with the same problem as mine.
It's caused by the rear lens shroud being too long. I contacted leica in the UK and they have not encountered the problem before.
I compared my Summilux with two others my dealer had, his mounted OK. The shroud on the back of my lens is about 1/2mm longer than on the two he had. My lens serial # is 30515xx Canadian, dealers were 26xxxxx and 28xxxxx.
All is not lost though, the complete lens cell unscrews from the front of the mount giving access to three small screws that retain the shroud. If you remove these and the shroud you can machine down the shroud. A competent lens repairer should be able to do this for you. It's important to mark the shroud and lens cell first so they can be re-assembled with the original orientation since the shroud has a cut out to avoid fouling the rangefinder follower cam.
I have a similar problem with a 35mm f1.2 Nokton. Of course, not being a Leica lens it's of no concern to Leica. The cause of the problem in this case is a bit more subtle.
The rear of the Nokton has two "fins" which protect the lens. Their diameter is just small enough to clear the semi circular cut outs each side of the throat in the camera body. However clearance relies on the "box" (equivalent to mirror box on SLR) being symmetrically placed about the lens axis. On my M8 it's fractionally off center which causes the edge of the box to bind on one of the fins
There's a small ridge at the back of the semi-circular cut outs in the box that the fin catches on.
I've had the rear edge of the fins on the Nokton machined off but it's insufficient so it needs a second try.