We have the same problem when trying to get Esri on premise working in Lumira.
When I look at the data traffic through Fiddler I can see that lumira is trying to reach http://OurArcGisServer/portal/portals/home
Even though our ArcGIS server is located at OurArcGisServer/portal (which i've entered in preferences in lumira) it seems like lumira adds /portals/home at the end of the URL.
When I enter http://OurArcGisServer/portal/home in preferences for the geo map I can see that lumira tries to reach http://OurArcGisServer/portal/home/portals/self (????)
The problem for us definetly seems to be that our arcgis on premise server isn't located at /portals but simply at /portal.