I've had the same problem for IE8. After some testing I found that the error occurs when in:

mwc.childNodes[i].style.height = (el.offsetHeight - bh - th) + "px";

the outcome of (el.offsetHeight - bh - th ) smaller than 0 is.

I replaced the code with:

var sum = el.offsetHeight - bh - th;
mwc.childNodes[i].style.height = (sum < 0 ? 0 : sum) + "px";

I honestly don't know whether this is the correct solution, but the error disappeared, and I've (so far) not seen any wrong rendering.

I remember I had the same error also somewhere else in the script. There I did the same <0 check.

Piotr

PS: Steffen fyi: It is pretty hard to comment without triggering the spam filter.