The problem does not lie in the redirection process, but in the application association. The 404 page is normal because a 404 page does not open a page, but initiates an application association. Due to compatibility reasons, the Android system cannot associate with Google Home or Assistant. Just refresh the page or switch the default browser to Chrome.
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This does resolve it for me, was using Android with duckduckgo browser and was getting the 404 error, switched the default browser to Chrome, and it worked at first attempt.
Thanks It works for me two.
I tried the same for a deezer connection from google home which was broken two, and it works equally.
It seems that google is on war with Firefox. 
You are my hero! This turned out to be my problem. I just so happened to switch ISPs, which meant a new IP address, so I was careening down the rabbit hole of some weird IP issue, when I realized that, upon switching my Cloudflare DNS entries, I just so happened to enable the Bot Fight Mode. Disabling it brought my Google/HA connection roaring back to life!