I've been looking into the error message and it seems to happen only when I have "footsteps" applied to a biped. The other issues of overlapping keys dosen't seem to be happening in 3ds max 6 or 7. Check to see if your animation is using "footsteps" and look in the "character studio user reference" menu about editing or extending "footsteps". "Footsteps" in 3ds Max are a kind of overiding animation in that if they are used in an animation they affect all keyframes added later. For more specific information on bipeds and footsteps check out:
http://support.discreet.com/webboard/wbpx.dll/~3dsmax
I hope this helps. A