IanM I regret to report that after updating the problem persists. I think it is related to the same problem that triggers the basic panel not being displayed. I've tried the usual: clearing flarum cache, server and browser cache, disabling extensions...etc. But the avatar is simply not stored, I have looked at the path where avatars are stored and it is correct, /assets/avatars/ (permissions are also correct); however when inspecting the avatar image to be uploaded, although it is true that it assigns an ID (name.png) and correct paths, it gives a 404 error.
About this update, also comment that some basic extensions like FoF Merge stopped working, they will remain active, but the controls will not be shown in the frontend.