The default port of HTTP/Apache is Port 80.
Since this Port is already used on my computer i installed Apache on another port.
URL then looks like: http://localhost:12345/
where 12345 is the port i used instead of 80.
if you use port 80 you don’t have to write ist, you just can write http://localhost/
With any port other than 80 i had the problem described above.
As soon as i switched to port 80 it worked fine.
I just ionstalled BuddyPress on a Server without different Port and it works fine there.
So i guess that is what causing it.
What the bug is exactly, that i don’t know. but it has something to do with the port i think.