Hey Jonathan, sorry for the late replay (Israel is having double trouble - Passover and COVID19)
nevertheless I was only able to fix this by reinstalling the arcGIS server component (someone had installed it for me about a year ago and it was not working since...)
anyway, I have asked all of the developers to preform backup to their tails and folders. made a full database backup and then removed (uninstall) the web adapters, uninstall the portal and uninstall the ArcGIS server.
then I have renamed all of it's leftovers (c:\arcgisportal Etc.) and reinstall everything back.
I made sure that all of the ports in the FW are open and follow the instillation guide.
I have tried to avoid reinstalling as much as I've could but eventually there was no way around it.
Sorry that I don't have better news for you...
Stay safe
Roy