Login page ERROR: Cookies are blocked due to unexpected output

Question:

I have a WordPress website that I recently discovered had this error on the login page:

ERROR: Cookies are blocked due to unexpected output. For help, please see this documentation or try the support forums.

I can only access the login page from wp-login.php, since /wp-admin/ displays a blank white page.

I've searched the web and tried many things with no success:

• Removing ?> and space at end of the functions.php
• Ensuring both functions.php and wp-config.php had no spaces at beginning or end of file
• Opening wp-config.php and functions.php in Notepad++ and converting to UTF-8 (people recommend 'without BOM' but that is only an option in the newest version of Notepad++ - I'm guessing UTF-8 does that now)
• I've set WP_DEBUG to true and have received no errors in debug.log, except for 2 that relate to when I hadn't fully uploaded some files. That particular error mentioned nav-menu.php; this confirmed that it is reporting errors, just not related to my login issues
• I renamed each plugin folder with _tmp at the end to try and rule out any plugins causing the problem
• I also renamed the plugins folder to rule out all plugins at the same time

Given these things I've tried, and that I'm not receiving any PHP errors, what else could I try?

Answer:

As per my opinion this related to some theme issue. Follow below steps and I hope this will your solution.

1. WP_DEBUG to TRUE set the value of WP_DEBUG to TRUE in your wp-config.php file. This will give you a meaningful error message on the wp-login.php page. After solution you can rollback this change.

2. See ERROR
Once you set TURE for WP_DEBUG, you may get Cannot modify header information error message. Based on that message, we can assume that there may be issues with your Theme.

3. Rename theme folder
If point 2, You need to rename your active theme folder name by some value(i.e. mytheme TO mytheme1). You should be able to login though it will default to the current default theme.

4. Active theme again
Once done step 3 and login success, you can again face same blank page issue. Don't worry because this is due to point 3. Active one of the available theme from Admin panels > Appearance > Themes.

NOTE: Don't select your old theme(i.e. we renamed it into point 3) because that theme has got problems. Once you solve that problem you can again use that theme.

For your theme related issue, you can check what is the problem by checking log file.

If still facing same issue, it means that you need to try other option like:
1. You need to update your wordpress version. Check how to
2. Inactive your all plugins, and again activate it.

That's done! Hope this help you well.