Stuck on Login Page after upgrade (2.1 -> 2.6.3)

After upgrading from 2.1 to 2.6.3 I’ve been stuck on the login page. The first time I logged in to 2.6.3 after the upgrade I did see the database tables need updating message, but since then I’ve been locked out, which has meant that for a while I’ve not been able to post on my site: http://www.32sixteen.com.

I want to post again, and after upgrading another site from 2.1 to 2.7 just the other week would like to upgrade again from 2.6.3 to 2.7.

I’m after anything I might have missed trying to get access to my 2.6.3 admin pages before I restore the old site from backups (yep, I’ve got backups) and try a 2.1 -> 2.7 upgrade. Unless of course that is going to be the best way forward.

I’ve read or followed suggestions on the following threads, which I’ll describe in more detail below:

http://wordpress.org/support/topic/226628
http://wordpress.org/support/topic/224318
http://codex.wordpress.org/Resetting_Your_Password

What I’ve tried so far

I’ve tried most of these mechanisms on Mac with Firefox, Windows with IE and IE, and Linux (Centos) with Firefox. Cookies and javascript are enabled.

Clearing browser Cookies
As on this thread: http://wordpress.org/support/topic/224318.

Result: Still can’t log in.

Deliberately Incorrect Username and/or Password
I deliberately used an incorrect username.
Result: correctly informed that the username is invalid.

I deliberately used the correct username with an invalid password.
Result: correctly informed that the password is invalid.

Correct username with correct password.
Result: Returned to the login page with no error messages.

Password Reset
Result: I got the email but still can’t login. The old password now returns an error message, as above; the new password appears to be accepted.

emergency.php
As recommended on the http://codex.wordpress.org/Resetting_Your_Password page.

Result: I got the email but still can’t login. The old password now returns an error message, as above; the new password appears to be accepted.

Checking the ABSPATH

I added the following line to the bottom of wp-config.php
echo ABSPATH;

Result: The correct absolute path to my htdocs directory on the server showed up at the top of the login page. Still couldn’t login.

Setting WP_SITEURL and WP_HOME directly
As recommended in this thread http://wordpress.org/support/topic/226628

Result: Still locked out.

Creating new database tables

As a completely whacky thing while in my wp-config.php file I changed the table prefix, from the standard ‘wp_’ to ‘nick_’.

Result: Success, of a sort. I was able to log in, and WordPress created a new set of tables with the prefix ‘nick_’ and the default ‘Hello World!’ worpdress post. All my plugins still show up, and if I go to the backup plugin I can see that my original tables, prefixed ‘wp_’ are still there. So I can login, but all my original content is locked away in other tables.

On changing the table prefix back to ‘wp_’ in wp-config.php I’m back to the infinite wp-login.php loop.

Reverting WordPress to 2.1
I reverted my files (not database) to 2.1.

Result: Still can’t log in – presumably because my database tables have been upgraded from 2.1 to 2.6.3.