I came across this issue today where I was getting an error when trying to add the <Fb:page-admin-edit-header/> to a canvas page of a page-only app.
The app was installed on a Facebook Page of which I was an admin.
Basically I had the following situation:
 canvas page that had <Fb:page-admin-edit-header/> rendered correctly at top when the canvas page was first hit
 canvas page had a simple non-Ajax form that submitted back to itself via POST
 POST url just contained URL of page
When I submitted the form, I kept getting the following error:
Errors while loading page from application
fb:page-admin-edit-header can only display on a fbpage application.
There are still a few kinks Facebook and the makers of Accountability Journal-cp are trying to iron out. We appreciate your patience as we try to fix these issues. Your problem has been logged - if it persists, please come back in a few days. Thanks!
I verified that the app was indeed a 'page-only' app, and that I had admin rights to the Facebook Page on which it was installed.
It seemed to me that this app was working perfectly until today, though I may be wrong about that...
It turns out that you MUST add the 'fb_page_id=[pageID]' URL param to the POST URL. Even though all of the FB variables were present in the original POST (fb_sig_page_id, fb_sig_is_admin=1, etc.) it appears that you must still provide the URL param in order for the FBML processing to insert the <Fb:page-admin-edit-header/> correctly.