Are you logging in using the secure https:// log-in? see http://en.support.wordpress.com/https/
Are you using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html
If you are not running one of the browsers and versions listed at http://browsehappy.com/ please try upgrading or switching first.
If you are running one of the browsers and versions listed at http://browsehappy.com please do these four things:
1. Try clearing your browser’s cache:
2. Make sure that you have third-party cookies enabled: http://en.support.wordpress.com/third-party-cookies/
3. Try with all browser extensions or add-ons temporarily disabled.
If it’s still happening, let us know which browser (and version of it) you’re using and post any error reports you get.