Hi Dan,
I was in a hurry yesterday. Now a bit more precise answer.
According to my experience the SNC-Name has to contain the domain name. So please try to change the parameter snc/identity/as and the SNC name in SAP GUI to p:CN=SAPServiceSBX@MYDOMAIN.LOCAL
Do you also have the Secure Login Client installed? Please have a look where the environment variable SNC_LIB is pointing to.
Please also have a at look at the implementation guide
Regards Thomas