Thanks to everybody who lent their advice to this issue.
I just wanted to say that although I never was able to determine the exact
reason the "get URL" was returning an empty string and empty result, using
the load command seems to get around the problem. I would be interested to
know what using libUrlSetLogField would reveal, but I didn't see Dave's
suggestion until after I had rewritten the script to use load instead of
"get URL". (I can't reproduce the problem locally and I don't want to bother
my customer to help satisfy my curiosity.) In any case, I just wanted to let
people know for the record how the issue was resolved.