Error on submit content type

Enabling AJAX form submit on a custom content type, causes the page to refresh and gives following error:

notice: ob_clean() [ref.outcontrol]: failed to delete buffer. No buffer to delete. in /var/www/web378/html/dev6/sites/all/modules/ajax/ajax.module on line 329.

The functionality kind of does work, because the Preview button works perfectly, but not the Submit button.

To clarify in advance: I am using the Form Block module on this custom content type, which lets me submit the post from a block inside my sidebar. That is why I don't want the page to refresh upon submission.

To save you some time, line 329 of ajax.module reads:

ob_clean();