Hey Joe:
I'd look at the browsers being used on the two different OS boxes?
I assume both have IE? Tried Chrome or?
Compare the IE settings?
Don't use compatability mode in IE. Apparently that makes IE want to behave like ver 7 and Portal only supports IE 9 and up.
You're logging into Portal from Win Servers rather than from desktop OS like Win7/8/10 , right?
I know I had to spend some frustrating time tweaking the various IE settings on our WinSrvr 2012 R2 server that is hosting Portal/AGS/DS when logging in directly from that server using IE.
IE on WSrvr2012 is setup differently than a desktop version, more control over security, etc...
Unfortunately, I was in the try all sorts of things mode and didn't keep very good notes on what I did.
If you search here for issues related to browsers and/or url doubling etc... you should find some threads that might have useful info.
For example: Portal 10.4 Request.Path error on IIS
There are some other threads about AD/IWA also that might have useful info?
Unfortunately, it seems that info on the inner workings of Portal (config files, etc...) can be hard to come by.
I suspect this is on purpose to keep us from royally screwing things up. Portal is tricky enough when just working through all the provided interfaces. I can only imagine the support headaches if (when) folks start digging deep and modifying things.
Maybe also check your portaladmin and ArcGIS/admin web sites and look through your settings?
FWIW: We do have our Win2012 servers letting us log into Portal 10.4
The Test server is tied to IWA and we can get in that way. However, we don't have ADFS (yet) running so that might be a difference in our's & your's setups.
good luck