Why isn't the user #1 able to edit the user details? I have one site in Drupal 6. When I log in as user #1, and I visit the page at user/[uid]/edit, I am not able to access this page. I get an access denied error.

Can any one please tell me what may be the problem, and the solution?
This is probably a conflict/misconfiguration of some module (or permissions) you have installed on the site and that deals with users or permissions.

Super user (uid: 1) has all privileges, whether or not the admin role exists.

Make a dev version of the site and start disabling modules one by one to find which one caused this.

Which modules do you have installed? Any custom modules? Is there anything in the database/error log?