I've got 3 RT-N66U's at my clients setup as APs and this problem is driving me nuts. They're great routers and so far no issues with them but it's embarassing to keep asking employees to reboot them for me manually after each firmware update (last one to 220.127.116.11.382_50470) as I am working 500 miles away. I've tried everything, clearing browsers' cache, different browsers, different OS platforms, on-site PC via VPN and no joy - am getting all the time this nonsense message until someone physically pulls the plug then they're back to normal:
The connection has timed out
The server at XXX.XXX.XXX.XXX is taking too long to respond.
The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer's network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.
I've even tried scheduling autoreboot which didn't help.
So, the answer is no solution for original Asus firmware affecting no only this device but others unless you use third-party firmware such as Merlin, Tomato or WRT depending on which hardware version you have. I have not tried the telnet or ssh yet as I cannot access the routers atm :/ Will edit and update when find a solution.
Try enabling telnet first, updating firmware then using telnet log into the router 'telnet YOUR_ROPUTER_IP' then use the same admin username and password used for web interface, as soon as you're in issue 'reboot' command and hit enter. I just tested this command and managed to reboot one of the routers that was manually rebooted by staff after fw update so it might work even though ACP isn't. Don't forget to turn off telnet access if you don't use it (for security reasons).