In several forums that use Discourse, the letter avatars have disappeared a few days ago. Since I have an Acc at Discourse Meta, I did some research there. The answer was found quickly.
The Avatars moved. Reboot your server (or just the discourse container). If you search here are several topics about the issue.
Apologies for the belated response, and thanks for researching the issue. We’ve rebooted the server and we’ve upgraded the version of the software as well.