Hi, using latest version of your plugin, we found two problems:
1) It is not possible to delete categories. It is probably caused by missing code global $wpdb; in the beginning of admin_event_categories.php file.
2) Using utf8 characters for category descriptions. You can try it adding "Žlutý kůň" as category description. It is probably caused by function htmlentities2() which does not support utf8 encoding.
Using following code works well:
$category_desc= htmlentities($_REQUEST['category_desc'], ENT_QUOTES, 'UTF-8' );
Thank you for your help...