There are already an undo and a redo button in the pixel editor.
The 400 error is caused by one of the values you entered in the upload form.
The beta codes were a few strings he posted on the website (utk.io) that were necessary for registration. Now sign up is open for the public, so the beta codes are no longer needed. They were just in place so that only a certain number of people could register, this way they could test everything and keep it under control while getting real user input at the same time.