I was having the same problem with the gray screen not responding on my desktop computer, so I went to pay using my phone. When I did this it complained that my zip code was not correct (which it was). I have a POB and the mail systems like to add additional digits after the zip code .
I suspect that this zip code error was the problem with the other page in my case.
Try a cell phone if you have it and/or if you have a POB enter the -XXXX after your zip code.
