I have the same issue as the previous user, and switching to Chrome does the trick
Viewing post in Battle Healer Hildegard comments
On Firefox and Chrome there's a 403 error
The resource at “/html-callback” was blocked due to its Cross-Origin-Resource-Policy header (or lack thereof). See https://developer.mozilla.org/docs/Web/HTTP/Cross-Origin_Resource_Policy_(CORP)#
There are a lot of error messages in the console, specific to Firefox, that all read like this one:
ERROR: shaderHidden/Internal-GUITextureClipText shader is not supported on this GPU (none of subshaders/fallbacks are suitable)