CTT Error connecting channel: BadInvalidArgument (0x80ab0000)|OPC Certification and Interoperability Testing|Forum|OPC Foundation

I am new to OPC UA and I apologize if the solution is obvious. I am trying to use the CTT for pre-certification testing, but am having problems getting the tool to connect to any servers. I am using a very simple python server as a starting point before I try to test more complicated systems. The code for the server I am using is here: https://github.com/FreeOpcUa/o.....example.py. I have verified that the server is running, and I am able to connect to the server with a python client, and with UAExpert client. I have disabled all firewalls and everything is running locally. 

I am following the Quick Start guide on the CTT help page. I've set the Server URL and UserAuthenticationPolicy in the CTT project settings. When I try to connect to the server by pressing the "Open Browser" button on the CTT project settings page, I get the error "Error connecting channel: BadInvalidArgument (0x80ab0000)". I also get the same error when I try to run any tests (OpenSecureChannel(MessageSecurityMode: None ...) Result = BadInvalidArgument (0x80ab0000)). 

From my server logs I am able to see that the CTT attempts to connect and 4 messages are sent between the server and CTT. There is a Hello Message, Acknowledge Message, OpenSecureChannelRequest and OpenSecureChannelResponse, none of which seem to have any errors associated with them.  

I appreciate any help, thanks