Strangely, it works if I add www. to the domain :
<img src="https://www.google.com/s2/favicons?domain=gbif.org"> =>
<img src="https://www.google.com/s2/favicons?domain=www.gbif.org"> =>
For other sites, I don’t have to specify www.
My problem is therefore less crucial, since I have an alternative.
I’m not an expert on domain names, but if anyone has an explanation as to why it worked yesterday but not today, I’d love to hear about it!