I resolved it myself, finally.
The following:
<?php get_header(); ?>
<?php include('templates/page.php'); ?>
<?php get_footer(); ?>
would be invalid. I instead overwrite the include line with the whole content of the file that was supposedly to be called. and voila.
It appears that comment.php needs info fetched when the header is called by get_header().
Sidenotes: this appears to be a theme specific bug, not a core bug.. But why would the default theme fail too? (at least on my case)
