Is it seated ok?
I ask because I did a quick change of oil on an exc I was running in an enduro a while back.
Got thru the 1st loop in great time, stopped at my base to grab a drink and gas up and noticed oil spewing from the cover. I'd nicked the rubber ring when I refitted the cover. Pissed me off big time!
Told another rider the following week who said he'd once done the same thing, it only takes a tiny nick and it won't seat properly.