I had just another problem. Well, happens from time to time :-) And as it looks like I am not alone with it. There are more user with this problem.
I’ve tried to install a WordPress blog (v. 188.8.131.52) on an 1and1 webspace and got an “500 Internal Server Error”.
The solution is easy, but usually you’ll not find it by yourself, because it is very unusual.
Add the following code to the .htaccess and the blog should work.
AddType x-mapp-php5 .php AddHandler x-mapp-php5 .php