500 error when switching between WYSIWYG and HTML

It seems that the info and changes I make are saved (can't tell for sure... but seems that way). But I always get a 500 when I switch between the two. Yet when I click the back button it goes to the right place. But I can't even be sure it goes to the right place because of the 500 error.

Any ideas?

I am on DreamHost, and I am not in Sandbox.

Thank you.
PS Great Script