Whenever I upload a HTML file, and go to the web address, it returns a 404 error every time. I was having an error yesterday with it saying ‘Too many redirects’ but I fixed that now.
Here’s the .htaccess file. The rest of it is some Mozilla Observatory recommendations to get myself an A+ rating for my site. I believe lines 6 to 9 is the code to remove the .html from the URL:
Try temporary removing lines 5-9 in the .htaccess file (take a backup first), clear your cache (and Cloudflare cache if you use Cloudflare), than try and load https://mazer800.xyz/youtube.html and see if it works. If it does, one (or more) of those lines you removed us to blame.