Error pages not working

Hello,

I configured some settings to make custom 404 and 50x error pages but if I type anything then the browser just simply reloads the main index.html.
For examle:
Let my website be webpage.com. I typed in 'webpage.com/this/does/not/exist/so_is_it.notavaible' it returned index.html without CSS...

What is the problem? How can I fix it?
(I run Nginx 1.16.1 on Docker, OMV, Debian 9)

Matt