Hi Thomas,
if you want a SPENGO authentication with the Secure Login Client you use the wrong stack.
You can use the default SecureLoginSPNEGOStack.
1. goto the Secure Login Server Console
2. search for your authentication profile you use and goto edit mode
3. on the "User Authentication" tab below, choose the SecureLoginSPNEGOStack as policy configuration .
4. press save.
best regards
Alexander Gimbel