Another suggestion — once you sure sure you have the correct IP, try a different browser (for example, if you normally use Firefox, try Opera or Safari or Chrome). Or if you have another computer on your local network, try accessing the device from there. This will eliminate any issues you may be having with plugins or add-ons that you've forgotten about, or maybe even firewall issues on that specific computer.
The first thing you should see when you access the device is a login prompt, which in certain browsers might be considered a popup. And also, you must have JavaScript enabled (come to think of it, if you don't have JavaScript enabled you'll have exactly the effect you're seeing - the unit will appear non-responsive). That last line was incorrect, sorry about that, and thanks to QBZappy for pointing out my mistake.