The valve covers have been known to warp.
Even a new valve cover gasket will not seal the cover to the head properly and a small leak usually at the rear adjacent the transmission dipstick/filler occurs.
Warps are generally caused by overzealous tightening of the hold down bolts - generally done at non MB specific service centres by persons not using a torque wrench.
It's easy to pinch the little half moon section at the rear of the valve cover gasket when installing it.