I'm trying to save an animation and no matter what setting(s) I use I either get a "This file isn't playable" error or "This item was encoded in a format that is not supported"

I've read that I should use a different player but neither of the windows players work and I really don't feel like installing another player or installing a codec. I've done several other animations and have not had this issue, same machine.

Any idea what has exploded?