While working on a WordPress site I noted several reports of login issues. The odd part was I was able to login and manage without any problems. First I had to reproduce the problem. I opened a new browser and tried signing up with a new username and pass. Then I attempted to login and sure enough:
Cookies are blocked or not supported by your browser. You must enable cookies to continue.
I Googled the error and after reading many posts I did finally find a solution. This site was using an older theme and according to the post at wordpress.org the following line is the cause:
<input type="hidden" name="testcookie" value="1" />
Remove it from your theme files and you are good to go. I hope our members can benefit from the post and I can always find it if I need it again 
http://wordpress.org/support/topic/cookies-are-blocked-or-not-supported-by-your-browser-1 Thanks WP!