Xalan fillbuf error

I've run into a mysterious error that I seem to be unable to track down.
It looks like this:

I'm running Xalan Java 2.2.D11. This is using a script which I've used
many times before, and which hasn't changed. I've even used the
script successfully on essentially the same data set, though I did
simplify the input data somewhat before the most recent run. I've tried
running on different machines with more memory with the same
result.

I've managed to isolate the problem to one section of my input data,
but when I try to isolate it further, it seems to get even more squirrely:
I'll get an error, comment out a section, get no error, uncomment the
same section, and still not get an error, uncomment something else,
and get the error again: it seems quite inconsistent.

A Google search on fillbuf found one person who had similar problems,
but no solution. I did find some references which suggested that fillbuf
had something to do with a buffer which should have had data but did
not, but I'm not sure what to do with that if anything.