There's a coding flaw in AdaptiveQos_Bandwidth_Monitor.asp that causes it to error out. Sometimes it'll work just fine (such as right after you reset your router), other times, it'll fail out. I've noticed it most often after I've gone through and named all the devices on the network. Unfortunately, I'm not familiar enough with the code yet to explain why that has anything to do it with it.
I wrote up a summary on it earlier in the week, but didn't post it here (emailed since it included images and such).
Asus has done a number of fixes to that page in upstream code, I'm waiting for a new GPL to merge their changes back. That's why I didn't spend any time tracking down the issue on the current version.