Ok my first step would be to compare a stock admin/orders.php against the one you're currently using.
Secondly the edit page does have the means to access the email service so the error message is telling you your email settings are not correct, or there's an email stuck somewhere.
Can you list the steps you take to get this to happen?
Log into Admin-> Click Processing-> Click an Order-> Then Edit...error
The same happens when i do it this way as well
Log into Admin -> Click Customers -> Select Customer -> Click Edit... error
As far as i know the 'processing' option is stock as i didnt load it and i have 3 sites and its on the all.
In the Admin page bottom left of the screen you have a box with
Orders
Pending:
Processing:
Dispatched:
Update:
