The 400 Bad Request error could only be caused by PHP code that specifically generates that error response. If you deleted all PHP code, that’s impossible, then there is no code to generate this error.
Then I’m not sure what else to try. Because your definitely seems to be working fine for everyone but you.
Actually, I just had a hunch.
I see your website is only a few hours old. As you can read in the orange banner in the client area: it can take up to 72 hours for new websites to be working everywhere.
It seems likely to me that this issue will just resolve itself within a few days at most. For a few possible workarounds to try, please see this article: