Hello,
Could you please check, if the policy configuration on the Secure Login Server for this Windows Authentication (SPNEGO) profile contains other login modules as the SPNegoLoginModule.
You can check that in the Netweaver Admin under "Authentication and Single Sign-On".
If you use a policy configuration with mixed login modules (non-interactive like SPNegoLoginModule with interactive login modules like BasicPasswordLoginModule), then the Secure Login Client will ask for username/password for the non-interactive login module,
best regards
Alexander Gimbel