For documentation’s sake, I would update this thread with my results.
Switching to just the stretch image brings with it chromium 72.0.3626.122-1~deb9u1
which had the same popup error for me.
Uninstalling chromium
sudo apt-get purge -y chromium*
and installing suggested chromium package (obtained from the link in this rpi forum post) did resolve the issue and now I have the heating page successfully loading.
Thanks for the help.
Aaron