Hi Dan,
I would like to understand your use case. You wrote that you want to implement SPNego for ABAP.
But your issue is related to SNC wiht SAP GUI. If you want to use SNC with SAP GUI and use your windows authentication (kerberos ticket) for SNC connection you need a Secure Login Client on your client workstation. Did you have it installed? Your error message looks like you are using another component for SNC connection.
Could you check on your client workstation where your SNC lib (SNC_LIB ) is pointing out?
KR
Valerie