I don't know if my situation is applicable in general, but after repeatedly getting the "Failed to render" error message when trying to save my kra animations as either a frame sequence or video (using ffmpeg), I switched to saving to my hard drive instead of my USB drive. That immediately solved the problem. Perhaps there is some issue with rendering to an external drive; that was certainly the case with my setup. I haven't received any error messages since.

Maybe that drive is too slow... I am sure not many people have tried to render to a usb drive!

Please see my posts here: viewtopic.php?f=139&t=152452

It seems to be an issue with rendering to a different drive and some other internal option and value settings being incorrectly set or badly passed around.