Hi,
Please check the general troubleshooting note: https://service.sap.com/sap/support/notes/1732610
and https://service.sap.com/sap/support/notes/1819808
Did you test the connection using IE or FIrefox. You need some configuration to allow SPNego in browser.
Check that you have the right ABAP patch to support SPNego.
If this did not help, please enable SPNego traces like described in SAP SSO implementation guide chapter 4.7.5.5 and the traces in Secure Login Library in chapter 4.8.1
The Implementation guide can be found in help.sap.com/sapsso
KR
Valerie