I would just double check you setting. It sounds like you're doing it right, but you might have missed a step along they way.
In your conversion of frame size, did you letterbox or crop the footage for 1920x1080?
Does FCP play them with the native render bar (dark gray)?
In FCP, what's the frame size? Pixel aspect ratio? Playback rate?
I've had errors like this and most of the time I've just ignored them. But it's always safer to figure out what FCP doesn't like about a clip.