Troubleshooting: Can't Login After WordPress Upgrade

Today, I was helping my client upgrading her WordPress-powered blog. He was using WordPress 2.6.x series. I upgraded using the standard procedur, and I thought the whole process was painless. But, it was not. I could not access its “Dashboard”. The login screen was there, but when I entered the username and password — I was 1,000% sure that I typed a correct combination — nothing happened.

Few months ago, I had a problem with the “Redirect Loop”. I made a speculation: removing all plugins under wp-content/plugins/ directory. Of course, after I made sure that I had the list of all active plugins. After that, I tried to login again, and voila! I continued re-installing all plugins needed. Problem solved.