2.5 Upgrade error: unable to go to admin panel!

My web pages load fine but when trying to enter the admin area I get the following error message:

Fatal error: Call to undefined function has_action() in /home/.sites/29/site109/web/wp-admin/includes/plugin.php on line 403

Now I’ve read here that it usually means a corrupt or missing file.
I’ve check sizes and reloaded these files three (3) times now with the same result.

Any thoughts.