Website going to 404 if browser has Japanese as secondary language

I am stuck at some strange issue looking for help. I do have a site which opens on all browser without any issue. But when we add Japanese language into Browser from Settings -> Languages -> Add Language

the site goes to 404.

After checking this in detail I found that when only Japanese language is added on browser then in request?accept-language?header?ja;q=0.8,ja-JP;q=0.7?added as extra lines. So I am hoping that server or WordPress is not able to handle the extra parameters in?accept-language?header. However any other multilingual site is working on my browser.

fetch("https://transcosmos.com/", {
  "headers": {
    "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7",
    "accept-language": "en-US,en;q=0.9,ja;q=0.8,ja-JP;q=0.7",
     ..... some more headers ....

  },
  "body": null,
  "method": "GET",
});

I have disabled all security plugins, removed any extra code on .htaccess, but it does not help much. I look forward to server support but they didn’t help much as the request is being already served by WordPress which we can see in second screenshot. Any idea what should I look for?

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