On the front side (where there are no connections) the cover can be pushed out. Now the inner part of the front is visible. Push the two retaining tabs outwards and remove the front part (it jams a little). Now you can slide the aluminium U-profile away from the remaining plastic part.

Sorry, I can't describe it any better. It's a stupid casing.

I am stuck at the second step... Unfortunately, I haven't found any more detailed descriptions or videos on the internet...

This is not possible, it is a so-called Sisyphus Box.