The freezing is something you will have to get used to. It sounds like you got it to work once, so it sounds like everything is set up fine. Just keep trying, it tends to take a few tries before you get it. Try erasing the cookies and browser data on your Wii U, sometimes that can help, there are a bunch of other methods of things people have tried that seem to work too.
One thing that I've done that really seems to help for some reason is to first open the browser, load a webpage, then quit the browser and enter the System settings. Then leave the settings and go back into the browser. Now power down the console and power it up again. After it re-boots open the browser and run the exploit again, after the exploit runs successfully then try GX2 again and see if it works. I know that sounds really weird, but it works almost overtime for me when I do that.
Hopefully you get it working! It just takes some time. Also, make sure to look through the stacked threads if you haven't already, there is a lot of great information up there that can help.