I don't think anything changed in Stardog that would cause a problem but I suspect something may have changed in your AWS setup. You'll need to enable inbound tcp on port 5820 in your security settings.
That of course was done. I added some detials to the question. I actually get to enter "admin:admin", but only once. If I restart the server, still 404. If I clear the cache, I get to enter the login/password again, with the same result.
Nothing in stardog.log