- Go to Windows\system32\inetsrv\urlscan\urlscan.ini
- Set AllowDotInPath=1, then save the file.
- In CMD prompt, type IISRESET
This also caused the WebAdaptor for ArcGIS Server to spin the candystripe progress bar and lag out, instead of finishing my login through the adaptor, although the normal ArcGIS Server login through port 6080 was fine.
One other hint was that the problem on Portal did cause an error on page with dojo.js
Hope this helps someone, could have used this to save me 8 hours to have this posted, so thought I'd put it out here.