Hi!
As mentioned before this can be due to the OS you are using, as mentioned I don’t know the specifics of it or if something is not set properly. I tested it on custom kali and it worked just fine, I just did the changes mentioned above and the browser loads as expected with any http site, even https sites load as expected. I didn’t get any 501 nor 404 messages and unfortunately it’s impossible for me to know what’s exactly going wrong as I can’t reproduce those errors. So maybe try to do some google research based on the exact OS used and the errors you are getting, also remember to flush iptables with the commands I shared some comments above to prevent iptables from improper redirections.

Greetings!
Diego