URLs get longer and longer

I finally got the component working now.

Well, now there is another problem, the URLs get longer and longer.
it should be www.domain.de/category/article and from the homepage to one different page, it works.
if i go on and click on another menu item the url changes to:
www.domain.de/category/article/category2/article2 and so on.
it just doesn't use www.domain.de as the base url, but always changes the base to the current url. why?
I tried to change the Rewrite base, but I get 500 Errors, if it's not RewriteBase /
It would be great if you could help me quickly.

Thank you, Ben Rau

mod_rewrite is of course acitvated and works. the .htacces should also be correct.