I'm experiencing the same problem with the control panel interface on a wzr-hp-g600nh with two computers connected to the LAN as well.
This was a NIB router freshly flashed. It worked poorly from the first minute I turned it on. I never had the LAN ports disconnected but I'm going to try ASAP to see if this makes the web interface usable.
And besides this, I notice that I get very slow speed for transfer files from one PC to another, over LAN.
But if I unplug the third PC from the router, the speed for transfer files from one PC to another backs to normal.
Not sure, but I think the third PC has a crossed cable.
I tried unplugging both LAN cables to no avail. Sometimes control panel tabs load, other times nothing. I've tried IE, FF, Chrome, and Opera... I'm looking into Konqueror but I'm starting to feel like it can't be a web browser issue.