Misfire and white smoke from engine?

[Tad2020]

Hi,

I have 2010 BMW 328i sedan.
I got 3 cylinders misfiring, according to Autozone code.
I saw some oil leak around the valve cover. I also saw oil on a few spark plugs: therefore, the problem was the valve cover gasket.

I replaced spark plugs, coil packs, oxygen sensors 1 and 2 upstreams, and valve cover gaskets. After replacing them, I started the engine. The first 20-30 seconds idle seems stable. After that, it starts making noise and misfiring sounds. And also I see white smoke coming out from under the engine. What is this smoke??? After replacing the valve gasket, I do not see any oil leak.

I opened up the valve cover to see again if there are cracks. I do not see them anywhere on the cover.

I have not changed fuel injectors.

What's the problem?