Fatal error after upgrade

Are you guys still stuck on this?

add_query_var is located in wp-includes/class-wp.php

Does that file exist? Does the function exist? (should be line 93)

Wes

I too have just upgraded to 3.1 and am unable to access my wp_admin all I get is a white page,my blog is ok and I was able to access before the up grade.
Have deleted all my plugins and re named my plugins file
https://www.wildflowersaromatherapy.com/blog/
https://www.wildflowersaromatherapy.com/blog/wp-login.php

The reason why some legacy installs cannot upgrade beyond version 3.0 is this:

Check wp-config.php for any lines that define a $wp-> object setting.

They will break 3.1 and above and give a $wp->add_query_var error.

For example:

$wp->cookiedomain = '.example.com';

Will work in 3.0 and below but break WP 3.1 and 3.2

You’ll have to research for the replacement define setting as an alternative to $wp->

For example, the modern replacement for the above line is
define(COOKIE_DOMAIN,'.example.com');