As the title says, I have a problem in functions.php (I assume it's in there) which causes custom fields to not be displayed on the new/edit post page, and when I add the meta box myself and save the data by updating the post, the data disappears.
However, if I save the data by clicking the Update button for the meta box itself (and do not click the post Update button), the data is saved and displayed properly.
I have posted the code in functions.php here.