Hi,
Internet Explorer is most likely causing the problem. It is something we're aware of, but Microsoft is unlikely to fix the issue. The good news is that there is a workaround for the problem.
Read this and follow the steps carefully. You may need to do this more than once, so you might want to bookmark that FAQ. It has screenshots to help you see what is supposed to happen.
Try that, and it that doesn't work, let us know.
The sad thing is that we're only seeing this happen in Internet Explorer. No other web browser is having an issue. I personally use Chrome, and the Firefox and Safari users I talk to aren't having trouble, either. Even Opera is fine.