I had got the same problem quite a few times in the last 2 months. Many people told me to adjust browser and security suite settings. I cleard cache, cookies and made sure browser ans secuiry suite had default settings. But nothing worked. Even just after formatting and installing OS and security suite , I was also not able to check data usage. I was using datafox to check my usage. 2 days back I had to format the pc and I noticed it working fine now. I noticed one thing that when I clicked on check data usage link just before logging in, I noticed the background image did not appear. Every time It did not appear, even though I could have logged in successfully but I could not check data usage. I think you are having the same problem now. If nothing works, may be formatting can do the trick