I can also easy reformat the server and reinstall everything, this won't hurt me, but I thought I might faced something strange here you guys should know about.
EDIT: It's also the first time I got this, I already changed the host domain on my productiv system a year ago and there it worked perfectly.
EDIT 2: I just found out if I type the IP instead of the domain name in the browser I got to the Login like it should be.