Did you keep your exsisting config.php file or upload the new when you upgraded... check your $gfx_chk is set correctly, you probably need to set to 2,3,4 etc..

$gfx_chk: Set the graphic security code on every login screen,
# You need to have GD extension installed:
# 0: No check
# 1: Administrators login only
# 2: Users login only
# 3: New users registration only
# 4: Both, users login and new users registration only
# 5: Administrators and users login only
# 6: Administrators and new users registration only
# 7: Everywhere on all login options (Admins and Users)
# NOTE: If you aren't sure set this value to 0

$dbhost = "localhost";
$dbuname = "root";
$dbpass = "";
$dbname = "nuke";
$prefix = "nuke";
$user_prefix = "nuke";
$dbtype = "MySQL";
$sitekey = "SdFk*fa28367-dm56w69.3a2fDS+e9";
$gfx_chk = 0;
$subscription_url = "";