After upgrading to Wordpress 4.1 and latest Atahualpa I get blank screen after login
 

I upgraded my site to WordPress 4.1 and in the same session I upgraded my Atahulapa theme to the latest version. Big mistake. Now after logging in to my admin account, I see a blank page and nothing happens.
The error.log in public_html contains the following message:
Call to undefined function hash_equals() in /home2/.../public_html/wp-includes/pluggable.php on line 1725
I don't know which upgrade caused the problem, WordPress or Atahualpa. And more important, I have no idea what to do to fix it, since I can't get into my Wordpress administration screen.
Is the only solution to revert to a backup?
Thanks,
--Alex

Sounds like an issue with a plugin. I would use from to move all plugins out of the plugin folder and see if you can get back in and if you can, then add the plugins in one at a time to find the problem