I've also noticed that conda is good at installing the current version of stuff, but bad at installing old/historic software. I'm not sure how the Anaconda repo removes old packages, but longevity may be worth validating.
Hey @cdiener, another approach would be to install the metapackage, which should really end up being the same as the environment file, but happened to work for me just now (it's also possible I have a cached numpy that is letting this solve).
That said, the fact that your solver is trying to pull numpy-1.18.1-py36h4f9e942_0 (python 3.6!) means something has really gone off the rails already.