misc.php fatal error

Hi guys, I hope you can help me with the following:

Fatal error: Cannot redeclare win_is_writable() (previously declared in /home/social2a/public_html/xxx/wp-includes/functions.php:1433) in /home/social2a/public_html/xxx/wp-admin/includes/misc.php on line 554

I get this message when logging in via the web interface (which displays basic html, not the usual wp login interface).

I believe I first got this error after updating WP to the latest version.

Thanks in advance!