The gearbox sump leaked on mine, I undid all the studs and then retightened but not overtight, just nipped using the smallest ratchet/ 10mm socket. Daft as it sounds, I think overtightening caused the leak.
What kind of gasket did you use for the sump?
Just taken my sump off and it has a thick rubber gasket in good condition. no leaks and none of the bolts were overtightened.
I was kindly given a new one made of cork. Not sure which is best. Maybe the rubber one.
Also maybe try some small amounts of gasket sealer on it and replace. Maybe it got crimped in one place.
I heard to apply some lube first so it slides into place and doesn't wrinkle anywhere as you tighten it.
apparently a blocked breather can cause a pressure build up
If you have an open filter breather can you not try to older system of using the vacuum to suck out the pressure from the crankcase?
I'm redesigning my circuit with a catch tank and filter but also under negative pressure (vacuum) from the airbox so hopefully it will help reduce blockages and overpressure in the crankcase.
Haven't tested it yet, got to rebuild my engine!