After a few days of trying, I found following answer:
As the HTML Editor worked godd on some places and not at home, I tought the problem lays by my configuration.
At home I have: ADSL-internetconection - modem/router with tree pc's connect to it.
I hav on every pc a virusscanprogram (AVG - Grisoft) and a firewall (Sygate personal)
I turned them all off. Nothing goes.
Finally I turned off the firewall of the router and you guess: Everything works fine, even after turning AVG and Sygate back on.
Hoping that will work for everyone with the same problem,