Thanks to both of you!
To answer all your questions:
Deactivated firewall: Yes, tried that now, no success.
Send delay turned on and off (in constellation with firewall on and off): Same result
No other modifications except GenTool. I even tried today to do a complete new fresh install, but also that didn't help...
I've tried it with port forwarding and closed all other ports again like described in your link Silent_Hunter, but that didn't help.
And additionally I've installed a VPN to see whether it works, but even there I get the same error (with firewall on/off, and send delay on/off)...
Any other ideas?
I guess the error is somewhere in my local installation or something is wrong with my account which I made here (just checked one more time, that I confirmed my email adress). That would be my next try, to see whether it works with another account...
Many thanks for any hint!