I recently ran into an issue where I couldn’t login to the admin side of my blog. Actually, I couldn’t even get to the login screen. Instead, I just saw a blank white screen when I tried accessing the Admin login screen.
Here is how I ended up fixing the problem.
1. I took this advice on how to locate what’s causing the problem. In my case, I’m hosting my site with MediaTemple, so I logged into Plesk > My Domain > Log Manager > and downloaded error_log. I recommend downloading that file rather than viewing it in a browser because it can be huge.
2. Scrolling to the very bottom of that file (most recent errors) I found this:
PHP Warning: session_start(): Cannot send session cache limiter – headers already sent (output started at /var/www/vhosts/thedeets.com/httpdocs/wp-content/plugins/all-in-one-seo-pack/all_in_one_seo_pack.php:4194) in /var/www/vhosts/thedeets.com/httpdocs/wp-content/plugins/wordpress-automatic-upgrade/wordpress-automatic-upgrade.php on line 227
so in my case, the All in One SEO Pack plugin and the WordPress Automatic Upgrade plugin both seemed to be causing problems.
3. Disabling plugins. It’s normally very easy to disable a plugin. Just login, click on Plugins, then click the disable link next to the ones you want to turn off. But what if you can’t log in? phpMyAdmin to the rescue.
Open your database using phpMyAdmin, click the wp_options table, then browse the contents for the active_plugins variable. Click edit. Select all and cut all of the content out of the option_value box. Paste it into a text file for backup. Click Go. You’ve now disabled all of your plugins.
4. Try logging into your site again. You may want to close and reopen your browser to clear the cache first. If one of your plugins was causing the problem you should now be able to login.
5. After logging in, click to your Plugins settings and re-activate plugins one by one to make sure things work for you. Do the ones that caused problems for you last. If you manage to lock yourself back out, go back to #3.