I have some more information on this:

Rlabkey works as expected from my laptop (both through labkey and in R terminal). It works on our production server through the terminal, but not through labkey.

I had been using baseUrl=labkey.url.base (which equals "https://xnight.primate.wisc.edu:8443/labkey/"). This results in an error saying 'cannot resolve host xnight.primate.wisc.edu'. If I use baseUrl="https://localhost:8443/labkey/", then the API works as expected. Does this behavior suggest any other things I should investigate?