Admin connection problems after update to 2.5

Hi,

This is not the first upgrade to the excellent wordpress 2.5 that I complete, but this one seems to have created a minor issue. I’ll do my best to describe what happend hereafter.

In short…
When: just after entering user/pass to access the wp-admin pages
What: Fatal error: Call to undefined function wp_register_sidebar_widget() in /home/myurl.com/www/htdocs/site/wp-admin/includes/dashboard.php on line 31

In more detail…
I upgraded from a 2.4.x wordpress to the latest 2.5 . I replaced all files (except the wp-config.php and /themes). I then ran the upgrade.php which completed successfully. The website is visible. The problem appears when I log in: I enter my credentials, submit them, and then get this error (nothing else on page). If I just go back to any post on the website and click the associated “modify” link, I can access the admin without any problem, no error to be seen. But when I access it directly at myurl.com/site/wp-admin I get the error.

I have no good hypothesis on where this might be coming from. I am willing to provide more details if this is of any use. Any help on the topic would be welcome.

Kindly,
Svenjick