I tried your model in V16 and I was able to save OK after making the new pose.
But when I tried in V11.1i, which I think you have, I got the error.
When I tried in V12 I was able to re save successfully.
Ballie_half_bones_05_v12.mdl
However, when I do a test model from scratch I can save successfully from any version. There is something unusual or corrupted about your model possibly. I know we were making bone hiding poses back in V11 and it seemed to work fine then.
I recommend you upgrade, but if your CD will let you run v12 (you can get the installer on the Hash updates page) you might try that.