Thanks for reaching out. I am not sure what the problem is.
sandbox.glowscript.org and it seems to be a server problem (403). There are no other console errors, only the 403.
You should also try to disable Brave Shields(Lion head icon) in the URL bar.
Thanks very much for the rapid response. It is indeed the case that disabling Brave Shields lets the program run.
As the developer of glowscript.org, can you offer me any advice on what I would need to do for programs to work even with Brave Shields turned on?