Being that you are in Europe parts for your particular tractor might be different (some EU tractor parts are different than US tractors), Rather a shock to see you are pinging in the center of Russia, I didn't think that was even possible.
Is there a local Kubota parts dealer?
Part numbers for what you want to do are based on serial number, so would need to know that to get you the right part numbers.
Your leak is very likely the seal for the propeller shaft for the 4wd.
it runs through the case just above the spit pin (Clutch housing vent).
You need to talk to a local dealer about the parts (have the serial number off the tractor) because I do believe there was an upgrade kit for that seal.
It's very very unlikely that it's the gasket leaking, could be the seal on the input shaft but that seal normally doesn't leak.
As far as repair goes I think ( but don't know for sure) that that seal on the propeller shaft will come out the front to be replaced. You DO NOT remove the shaft section that is in the transmission, if you do you will be tearing the whole thing apart to reassemble it.
Propeller shaft seal
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