Pat, I just tried to install the 5.6.1 (using the default OpcDaThing.config), and I could connect to my OpcThing (with my Matrikon sim). The only issue I had was with ssl connectivity (I'm looking into that now), but I can get it connected when setting the IsSecure flag to false. My AutoConnect is set to false as well.
I have identified the issue I had, so wanted to share and save your time.
When doing version upgrade of the OPC DA SDK 5.6.0 to 5.6.1, the host computer MUST be rebooted.
If not, the property "ConnectRetries" will not accept the default value "-1" due to it's definition as being a UInt data type.
Following a reboot, it works beautifully. :-)