This error is generated in wp-login.php , it happens if server is unable to set cookies, this can happen due to several reasons, one of the common issue is: output being sent before setting up of cookies.
Try out following options:
- Update WordPress core, if it is not up to date
- change theme, if you are using custom theme, just ftp into server and delete the custom theme folder, WordPress will revert to default theme.
- If issue still persists, rename plugin folder
Also check error log, errors may not be printed to screen.