UPDATE:Please see Part II, which includes links to the official WordPress support forum.
After trying everything from disabling plugins, to re-creating permalinks, to removing my .htaccess file, I still could not restore my permalink custom structure after upgrading WordPress from version 2.5.1 to 2.6. I then posted a new topic in the forums for help and continued to search for answers.
Finally, using several posts and a hack of my own at the end, I came up with the following procedure which fixed it for me:
1. Upload this file to your wp-includes directory:
Save this link as “rewrite.php” and then upload it to your /wp-includes directory.
2. From the dashboard, set permalinks to “Default” and Save it.
3. From the dashboard, set permalinks to “Custom Structure” of your choice and Save it.
4. The above fixed it, but I saw some errors at the top of each of my pages pointing to /wp-includes/plugin.php – so I added the following line of code which is a hack to suppress the errors:
error_reporting(0); Put this line at the top, just below the opening
That’s it. Please let me know if you have any problems by leaving a comment below.