I have been getting this problem on and off for several weeks. Often the left hand side navigation is still displayed. This morning the whole webpage went blank with just a single line "Internal Server Error". A forced page refresh did not resolve it.
One of our staff is having the same problem when trying to view a specific Discount. She tried using a Chrome incognito window and it made no difference. She has had this problem for many days. She does not get this error when viewing other Discounts.