The function get_comic_path has this signature with these parameters:
function get_comic_path($folder = 'comic', $override_post = null, $filter = 'default')
However, there are two places in stock ComicPress 2.9.x where an unneeded parameter is being passed in as the $override_post, and the actual post is being passed in as $filter:
displaycomic.php, line 103: $comics = get_comic_path('comic', false, $post_to_use);
displaycomic.php, line 197: $comics = get_comic_path('comic', false, $post);
The solution is to remove the second parameter:
$comics = get_comic_path('comic', $post_to_use);