If SNC is activated in SAP GUI and in NWBC, the logon popup never appears again.
This is exactly what you would expect from a
Single Sign-On technology, but sometimes you might still want to have a logon popup (for testing purposes for example).
If you have previously used the SAP Logon Pad, you might know the options SNC Logon with Single Sign-On (Enter) and SNC Logon without Single
Sign-On (Shift+Enter).
Unfortunately we do not have this kind of "switch" in NWBC.
Fortunately however there is a workaround:
You can use the existing external service (delivered with 7.40) /nwbc-no-sso.
This alias calls the NWBC service without support of certificates and always forces a logon screen.
Alternatively, you can create your own external alias in transaction SICF.
See: http://help.sap.com/saphelp_nw70ehp2/helpdata/en/78/9852bdc06b11d4ad310000e83539c3/frameset.htm