I think I solved the problem.  The script I was using would key everything on the figure, even blank morph channels.  It must have added something somewhere that Carrara can't handle.

I wrote a script that only keys the rotation parameters on all the body parts, and the translate parameters on the hip.

Now I can animate in poser with IK, run the script to key everything, switch everything to FK, import the file into Carrara, turn constraints off, and the animation will run just as well in Carrara as it originally did in poser, and the file saves and loads fine.

Thanks Disco, your idea of using simon with the walk designer gave me the idea that the script I was using was screwing up the file.