In version 1.2.39 of the Post Notification plugin, I found that on the settings screen (at /wp-admin/admin.php?page=post_notification/admin.php&action=settings) when I check Default categories ('The categories which will be automatically selected when a user subscribes') my selection was not displayed when I clicked the 'save' button and the page reloaded.
I think this is because the code between lines 701 and 704 of admin_settings.php is contained within short tags i.e.
<? $selected_cats_list = get_option('post_notification_selected_cats'); $selected_cats = explode(',', $selected_cats_list); ?>
So my server (due to its configuration) did not parse those lines as php (instead they were just sent to the browser).
To fix this problem, just change line 701 to:
I hope this helps someone,