Erni:
It was very strange. I could not access it from home, but if I took my laptop and went a couple of kilometers to a friend of mine I had no problems.
Problem should be solved. Just a badly assigned IP not routed by some (but not every) ISP.
It may happen again, please notify me (writing to webmaster@arduino.cc , I do not regularly read the forum) in that case.
Thanks grendle to help me collecting important informations usefull to identify and solve the issue.