Could the E-bike drivetrain wear down till needing replacement after 400 km?
I have tried to regulate the rear gear mechanism by adjusting the position screws but failed to make any improvement.


That's almost never the adjustment that the rear derailleur needs. Those screws are called the limit screws, and they set how far in and out the derailleur can move. All they do is stop the derailleur from moving the chain all the way off the cassette on either side. There's basically nothing that causes those screws to move on their own so, assuming they were set correctly when the bike was built, they should almost never need to be touched. The symptoms that do mean they need adjustment is that either the chain is falling off the top or bottom of the cassette (derailleur able to move too far) or that gear changing is just fine, except that you can't change into the smallest or biggest cog (derailleur not able to move far enough).

The correct adjustment is to use the barrel adjuster to get the correct cable tension. That very often is needed on new bikes, because the cables tend to stretch a bit after they're installed. Argenti Apparatus has already posted a video so I'll not go into more detail than that.