This problem has happened to me many times with my personal site as well as client sites. After trying different things and looking at other articles what worked for me was to:

• First log out using this url: http://www.yourdomain.com/wp-login.php?loggedout=true
• Rename your plugins folder to “_plugins” to back them up and create a new folder called “plugins” – this insures that there is on conflict with any plugins that are being run
• Replace your current wp-login.php file with a new one
• NOTE: You might get a header php error, and if you do follow this article posted at WordPress

Hopefully those steps can fix this problem for you. If something else works for you, feel free to post the results.

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