As a further confirmation, I've stood up a new 9.1(0.32) version of Unity Connection and tested both the CUPIFastStart and my own code and it works just fine. Something has definitely changed in 9.1(2).
I also did some further debugging of the ConnectionServerRest API code and found that the WebCallResult for the GetVersionInfo function returned the following:
the first item here is why are you running 3.0.38 of the CUPI SDK? That's from July of 2014. the latest version out is 3.0.58. It's easiest to juse use NuGet for keeping the libraries up to date (all my SDKs for notification, CUPI, ODBC etc... are available on NuGet).
Once you have that updated we can look at what's going on with your connection.