If I recall correctly, the lake right next to Leningrad was changed so that supplies could be transfered over it (as happened historically)... not entirely sure that's working properly or is the cause of your situation, but it could be.
Also, if you don't have Kiel *garrisoned*, they can certainly send convoys through there.
Also, if you don't have Kiel *garrisoned*, they can certainly send convoys through there.