This HTML / Visual button error has been around for at least seven months (and more than likey a lot longer).
I am currently trying everything that people have suggested and am running out of ideas.
Someone did mention that your host could be Gzip’ing files as they are loaded; as does WP – this causes problems, so you could try to switch off Gzip?
I am not knowledgeable with servers and .php files so appologies for misinformation.
k s c