It's a _bak file. I was getting bucket errors on the render, unless the cloud acceleration cache was set to none.
Ideally the cloud acceleration cache wouldn't cause these issues of course, but for now turning it off is an option to prevent the problem when it does occur. Fortunately in most situations the acceleration cache provides benefit fairly transparently and without detriment to the rendered result.
This occurs very seldom. Setting the acceleration cache to none works. Now that I know what causes this, I can avoid/fix the problem easily.
This is good to know, I have been trying to work around this by increasing the cloud samples with no luck of course. Thks