Poldi wrote: That was not the (only) wrong thing.
I used a new image -> removed some Groups -> enabled my sensors and GPIO -> configured GPIO (3,5,11,13, where my LEDs are connected). -> I saved the configuration and closed the browser (without logoff) -> logged in at the pi and did a reboot (sudo reboot).
After restarting the GPIO port was initialised (after powering on, two leds are lighting low, after starting was completed (login prompt) they were switched off).
But the server can not be reached.
Anything else I can try?