Accidentally found the solution. The checkbox was set to multiple and it did not compare the stored value to the one in the selectbox.
This line:
<option value="<?php echo $page->post_title; ?>" <?php if ( $page_selected && in_array( $page->post_title, $page_selected ) ) { echo 'selected="selected"'; }?>><?php echo $page->post_title; ?></option>
has to be changed to this:
<option value="<?php echo $page->post_title; ?>" <?php if( $page_selected == $page->post_title) { echo 'selected="selected"'; }?>><?php echo $page->post_title; ?></option>