Thanks, using the IP address does mitigate the problem. I can get it to consistently happen when accessing the web interface by name, is there anything I can do to help debug this?Access your router by its IP rather than by its name. A few users have encountered that problem, but I'm unable to reproduce it. Since the issue also exists in the stock firmware, this is something Asus will no doubt resolve eventually. They are still finetuning the new authentication system that they implemented a few months ago.
A POST to /apply.cgi receives a 200 OK response (my browser's traffic inspection is only capturing the response code and not the body unfortunately, and I haven't tried direct traffic inspection since this is HTTPS). It appears that some JS in the response loads /Main_Login.asp?error_status=6.