[Plugin: External DB authentication]problem signing with an external user

Hi I have a site with wordpress platform then I wing task of integrating an external forum but could share the user registry, I decided to use the platform MyBB and External DB authentication plugin, the plugin installed MyBB and decided after aver porB if I could log into wordpress with a registered user of MyBB, but I throw this error Fatal error: Call to undefined function login_header() in /home/maxterg1/public_html/wp-content/plugins/external-database-authentication/ext_db_auth_mybb.php on line 529 reload the page and I could magically logearme a MyBB user, I thought everything was fine then went back to record another user MyBB and try to log into wordpress just does not recognize the user no longer get the above error but I can not start, if anyone knows how to solve this problem and they would appreciate, Thanks for the attention