I first saw this happen after running the SUPEE security patches 1533, 5344 and 5994
When this began I opted to upgrade to 1.9.1.1 (I was on 1.9.1.0)
None of these helped.
I have display errors, developer-mode both on, plus logging enabled, but no logs or reports are created.
The Javascript error "Uncaught TypeError: Cannot read property 'value' of null" copmes up on the console and I traced it to the element frontend_input. The call to $('frontend_input').value retuns null as no form elements are being rendered.
So I started looking into the /app/code/core/Mage/Adminhtml/Block (and controller) files to see if I could spot the trouble.
So far I can only be sure that the form elements are not being prepared. I'm still tracking through the process and where it breaks. I can see the Form object is created, but I don't see the _setFeldSet function being called on it. This leaves all form elements null and that is where the javascript error is coming from as the script tries to engage with the form elements it expects, but are not there.
I can update the database directly and the attributes all work fine in the frontend...
I'll let you know if/when I track it down. Or anyone have any ideas?
Thanks!