I had this problem with my 521. For whatever reason, the cases don't seem to seal there...The first time I rebuilt it, it leaked. you cant hurt anything because the crank has 2 inner seals to seal each cylinder. Its just oil leaking for the rotary valve, water pump gear..ect. Anyway...What I did was orange gasket maker on the outside of the engine. It was the only thing I could do unless I wanted to take the bottom case off again...So I smeared some of that in the "crack." On mine there is a 2 or 3mm indent the whole way along the seam...I filled it with gasket maker and it stopped. Not pretty, but it worked.
This time when I rebuilt it, I made sure I put lots of crankcase sealer in that area. And even for a precation, I put gasket maker on the outside...
Funny you had this problem too...I just thought I had not put enough sealer the first time I did the engine...