400 Bad Request custom error page issue

Website URL

https://desmos-renderer.xo.je

Error Message

400 Bad Request

Other Information

I am facing some issues with the custom error page. The 400 Bad Request error page is not showing as I expect it to. I put my custom error pages through the dashboard in Error Pages section. But I get this:

It is not showing my custom error page. An error 400 shows a page like above instead.