I am in the process of upgrading a site from 03.00.13, with the plan to end up on 06.00.02.

I am following the upgrade path recommended at

Suggested Upgrade Path
http://www.dotnetnuke.com/Resources/W...

I have successfully upgraded up to 04.04.01, checking site functionality and logins at each step.

When I do the upgrade from 04.04.01 to 04.06.02, the update log shows the update completing with "Success" reported at each version step and no problems.

When I finally try to log into the site, now at 04.06.02, I get a User Login error: "Login Failed, remember that Passwords are case sensitive", which would normally suggest that something got fumbled in editing/copying the configuration/system.web/machineKey node in web.config. However, clicking on the site logo sends the browser to the site's main page and the menu displays as appropriate for the user whose login supposedly failed. Also, the USER skin object now shows that that user is now logged in.

I have recently upgraded another 03.00.13 site on the same machine (different DNN instance) to 05.06.03 without any problems.
I have, several times, reverted to a backup of the site after the upgrade to 04.04.01 was successfully applied and then reapplied the upgrade to 04.06.02 - each time with the same result.
I have, several times, gone back to the start with the site in its original state at version 03.00.13 and stepped through the upgrade steps. Each time, I get the same result after the upgrade to 04.06.02.

Also, it's not just the first user login that has this problem - every user login tried appears to fail, and then on subsequent clicks, the site updates to show that the user has in fact been logged in.

Any ideas?