I still don't know the reason but i-MetrO is broken again for some cities. I investigate the matter, as well as the guys from the hosting service (which is free, let's not forget that) but I have no idea when this will be solved. Sorry for the inconvenience.
One positive note though: I found out a mistake that prevented to use the service for Freiburg in Breisgau. I fixed it and it's one of the cities that work all right ;-)
[Edit] The service is running again this Tuesday morning (CET). Enjoy!