This made my day, thanks! Loopback worked. So I tried to connect the two RS232 to ttl module pins also directly without cable. Now it also worked.
But only with crossover! When using straight, it also does not work to change the rx/tx pins. Any ideas why?
I only need 1 - 2 m. I went for rs232 because in my last project I had several microcontroller freezes because of 20 cm cables near higher voltage cables. When changing them to pcb the problem was gone. Can I also be save by using the correct shielded cables?