I figured it out. It's a glitch that only appears when you use the AC68U firmware on an AC68R! I was using the AC68U 217 firmware posted in this thread on my AC68R and that's what causes the problem. When you are running the "U" firmware on an "R" router and you click that save button, here's where it tries to go:
http://192.168.1.1/Settings_RT-AC68R.CFG
That file doesn't exist so if you manually overwrite that URL with this:
http://192.168.1.1/Settings_RT-AC68U.CFG
It works and you can save the settings. I don't know if this is a bug and the "U" firmware is supposed to work on "R" routers... or if the "U" firmware was just not meant to run on the "R" routers. If the former, Asus will have to fix this bug. If the latter (they are not interchangeable), we'll have to always be sure to use firmware labeled "U" or "R" depending on which flavor you have.
Mike
thanks so much! I hadn't been paying attention to that but putting the url in manual as you said worked like a charm so that I could now save my settings. Good work!