OK, I tried some more firmware and just the original ones are working. I tried versions for 500b, 500bv2 and 500g. They all work when they are original. I flashed them with the restoration-tool and after reboot the routers web-access was available. If I flash any oleg-firmware (I tried b, bv2 and g here, too) and reboot, the router is no more available not even to ping! I used the routers firmware-update and the tool, but there's no difference!
Hoping for help!