Patrik sorry, you’re right, it did not work. I assumed that when the thumbnail loaded, the video would play as well, but that is not the case.
I have manually instructed Flarum to overwrite the Referrer
header with no-referrer-when-downgrade
for your forum, and it should work now – I clicked the video in the discussion you linked above and it started playing correctly (at least that confirms that the issue is with the extensions not setting the referrer properly).
I think it now shouldn’t matter if you use FoF Formatting with Lite YouTube Embed, or just the FoF Formatting extension.
Note that changing your custom domain will rewrite the config, so let me know if it stops working. Also, keep in mind that if you use VPN, or YouTube simply doesn’t like something else, the video might still refuse to play – at that point, I unfortunately can’t do anything as all of that is governed by YouTube’s (frankly, mostly unclear) policies.