Yesterday, I helped one of my clients with her WordPress-powered blogs. When I tried to find the solution — using search engines — I could not find any exact solution.
Problem overview
Okey, here’s the problem: When I tried to login to WordPress dashboard — not WordPress.com, but self-hosted — I always got these errors:

Redirect Loop
The browser has stopped trying to retrieve the requested item. The site is redirecting the request in a way that will never complete.
The browser has stopped trying to retrieve the requested item. The site is redirecting the request in a way that will never complete.

• Have you disabled or blocked cookies required by this site?
• NOTE: If accepting the site’s cookies does not resolve the problem, it is likely a server configuration issue and not your computer.

I’m not sure what caused this problem. I thought it was my browser (Firefox), but I got the same problem on other browsers.

Server problem? I don’t think so. I have some WordPress installed on the same machine. Here are some technical data about my client’s WordPress installation:

• WordPress 2.5.1 — When I upgraded from the previous version, it worked without any problems.
• Plugins should work, but it’s not. I have same plugins installed and mine has more plugins.

Troubleshooting: Disabling plugins, cleared the cookies
Okey, I disabled all installed and active plugins. Because I couldn’t get in to “Dashboard”, I downloaded all plugins, and remove them from server after that. You can disable plugins by moving all plugins (files and directory under “wp-contents/plugins” to other directory. It’s the same.
I also cleared the cookies. If you’re using Firefox, go to Tools > Options > Privacy and click on the “Show Cookies” button. Delete the domain cookies (if any). For Opera, go to Tools > Preferences > Advanced > Cookies. You can delete cookies from the corresponding domains from the dialog box. For Internet Explorer, you can find it somewhere in “Internet Options”. :)
I tried to login again, and still got the problems. Is it because of the older version of WordPress? Probably. So, I took the risk by upgrading it to the latest version: WordPress 2.6.2.
Troubleshooting: WordPress Upgrade
I just wanted to make sure that I had the database backed up. So, I created backup for the database, and uploaded the new files. It seems that I can directly upgraded my WordPress without going to Dashboard. Once you have your new files uploaded, just visit the Dashboard URL, or simply login to admin area from yourdomain.com/wp-login.php or yourdomain.com/wp-admin/. After the upgrade, everything was back to normal. Great!
The first thing I did after that was to re-upload the previous installed plugins and updated the out-dated plugins. When I went to plugin management page, I got a warning message:

The plugin ../../wp-content/themes/theme_directory_name/header.old has been deactivated due to an error: Invalid plugin.

Okey, there was an error because the themes needed a plugin that did not exist. Luckily, the theme was not active (being used). I simply removed it, and the warning message disappeared. After that, everything worked without any problems. Problems solved. Great!