Unrecoverable error upgrading 6.4 to 6.9 - Tried everything I can think off

Like others I have hunted up and down looking for clues as to why my upgrade failed with an unrecoverable error when running update.php.

Basically, the update.php shows the html code of update.php and clicking to the error page takes it back to the update.php page.

I want to upgrade from 6.4 to 6.9.
I read and executed the steps necessary. Take site offline, change theme to Garland, disable all contributed or custom modules.

IS THERE A WAY TO APPLY THIS UPGRADES MANUALLY. The site talks about it but can't find explanation on it.

I tried to look at the query logs ... There are lots of SQL statements there. Basically the last one updates the session table.

I started to trace the update.php code. I might still resume on it to see if it will help.

I was however able to upgrade a 6.4 VANILLA install to a 6.9 VANILLA install. This would lead me to believe it is a contributed module. I thought disabling them in a previous steps, allows upgrade to work on CORE only. But the sql statments show that there updates related to the contributed modules happening also.

I am using FRONTPAGE, DEVEL, Google Analytics, Meta Tags, Robots, SwitchTheme, tinymce, Webform, Captcha, Captcha pack, XML sitemap as contributed modules ... though they were disabled during the upgrade.

DOES ANYONE HAVE ANY INPUT?

Now I am thinking of uninstalling the contributed modules one by one and upgrading and see which one is the culprit?

Is there a faster way of finding out the root cause of this problem?

Thank you.