Newsletter doesn't get send out

The newsletter doesn't get send out and the only log information I get is this recurring error:
[29-Apr-2013 15:33:17 UTC] PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'codex_custom_init' not found or invalid function name in /home/noordzeenieuw/www/wp-includes/plugin.php on line 406

After upgrading to 2.0.0-RC5 I get these errors in addition to the above one:
[29-Apr-2013 15:37:47 UTC] PHP Warning: Creating default object from empty value in /home/noordzeenieuw/www/wp-content/plugins/e-newsletter/email-newsletter-files/builder/class.builder.php on line 607
[29-Apr-2013 15:37:47 UTC] PHP Warning: Creating default object from empty value in /home/noordzeenieuw/www/wp-content/plugins/e-newsletter/email-newsletter-files/builder/class.builder.php on line 610
[29-Apr-2013 15:37:47 UTC] PHP Warning: Creating default object from empty value in /home/noordzeenieuw/www/wp-content/plugins/e-newsletter/email-newsletter-files/builder/class.builder.php on line 611
[29-Apr-2013 15:37:47 UTC] PHP Warning: Creating default object from empty value in /home/noordzeenieuw/www/wp-content/plugins/e-newsletter/email-newsletter-files/builder/class.builder.php on line 612
[29-Apr-2013 15:37:47 UTC] PHP Warning: Creating default object from empty value in /home/noordzeenieuw/www/wp-content/plugins/e-newsletter/email-newsletter-files/builder/class.builder.php on line 613

I can't send any newsletters at the moment.