You seem to be having the same problem I am, if so hopefully I can elaborate on this. First, my code looks almost exactly like yours, and that is why I assume our problems are the same.
What I have figured out is that the richtext widget has a problem with the value field when ad_form is in display mode. To test this I commented(#) out the value field and went into the page, then I uncommented the field and loaded the edit page, and it worked fine, but only when the page is in edit mode. If it tries to go into display mode with a value in it you get a Request Error:
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Unknown text input format, ''. Valid formats are text/enhanced text/plain text/fixed-width text/html .
while executing
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Starts out like this if we are in fact having the same problem. Hopefully this will help.
