Did you try creating a random polygon with 3 vertices (click-click-finish) to recreate the error? I did that and it was fine, then went back to tracing to create the new feature. I traced half the feature, then the other half with no errors and then merged the two.
Had same thing happen twice on a separate brand new FCs that I unchecked the Z-values box for when I created it. Went to trace a line to create polygon and got the error. Tried again and didn't work. Might have something to do with vertices/lines that coincide for the same feature when it is created. That's just speculation.