I have two different gridfx controls on the page and they both exhibit this problem.
I use the design time delete functionality, however I do have an item deleted handler for each grid that databinds the grid. For some reason I had to do this, without the databind I get the following error on delete:
"Index out of range. Must be non-negative and less then the size of the collection.
Parameter name: Index"
With the databind in the item deleted handler the delete works fine, but upon the delete of the last item I get an error as described before.
The "failed to load viewstate" error occurs after the page load event, but before any of the other events I use (databinding, databound, rendering) are called.