As I continue testing all tracks and switches, I realized that engines cannot go through switch C4. I initially thought that the tracks needed to be cleaned further, but as it turns out, it is a power issue:
I tested the wiring underneath the table and it is all fine. It must therefore be the soldering between the wires and the switch tracks. After some testing, the closure rails are properly connected, but the stock rails have clearly an issue:
It looks like some soldering exercise for tomorrow ...