Please try the way below, it will fix the issue.
1. Create a new page with any name, in my example, I name it to “test_post.php”, and use the same editor code as your editor page. (only the editor code)
For example, at my end, I used the code below in it.
<?php require_once "richtexteditor/include_rte.php" ?>
2. Set property AjaxPostbackUrl for your editor (in the old editor page), like below. The url should set to the new page above.