Hi, sorry for the trouble you experienced! Are you possibly saving your file to a Windows share? That could explain part of this. Also, it sounds like the second time you saw the error it was due to the auto-backup failing possibly due to a permissions error? Console.app in Applications/Utilities might shed more light on this error.
Please email support at
omnioutliner@omnigroup.com if you haven't found any obvious reason for this answer and we'll help you get to the bottom of it.