Hi @lsw,
This is a new one for us. Could you provide the output to the following:
conda --version
and inside your environment:
conda list
Usually we see these issues when your system R starts interacting with your conda R, but that doesn't seem to be the case here. So I am wondering if a package is corrupt. You might try updating conda (conda update conda), cleaning your package-cache conda clean --all, and re-installing the environment (remove first with conda env remove -n qiime2-2018.2).