This can be caused by several things:
if the link is from within the Stadtradler site, you have found a broken link in my site, and this is my failure: please let me know what link in the Stadtradler site is broken, so I can correct this.
if the link is from a referring site, this may be caused by the referring site using a wrong address for the link: please let me know this as well, along with the info who is referring to the Stadtradler site, so I can contact the referrer and have them fix their link to the Stadtradler site