After searching the net, it appears that the generic "RENDER FAILED" message (below) appears when customizing certain views for a multitude of reasons, each with its own unique combination factors and its own solution. So, here is one more set of parameters / solution to add to the chorus.
I had a view that was grouping on a single field and also had to provide the AVERAGE of a number field. When trying this with the default Group By settings, I would get the <!– #RENDER FAILED –> error message.
The fix was obscure but simple. Change the Group By options to Expanded and the problem was solved.