Cookies are blocked due to unexpected output in Wordpress

I had same issue. What I did is I added ob_start() at top of wp-config.php then it allows me logged in. After that I update one of plugin that was requite to update. Then I logout and removed ob_start() and I surprised error was gone and never came again.

I tried following solutions by which similar issues get resolved

• Changing file formats (UTF-8 BOM to UTF-8)
• Renaming Plugins folder
• Replacing Files which are giving error (On enabling wp-debug)

But in my case final solution was to upgrade the WordPress, or more likely you can say replacing core WordPress files using FTP.I followed this guide for manual upgrade:http://codex.wordpress.org/Updating_WordPress

I had the same error and was not able to fix it with the answers here in this thread nor this thread helped me: https://wordpress.stackexchange.com/questions/208878/login-page-error-cookies-are-blocked-due-to-unexpected-output

My solution is:

1. Open your WP directory with FTP and download the wp-config.php file
2. Copy the information in the wp-config.php file
3. Open a standard text editor (I used the Windows text editor)
4. Paste the information you just copied, into the empty text editor.
5. Save the file as wp-config.php and choose ANSI as encoding!

6. Replace the new wp-config.php file with your exisiting one.

Done. This helped me.