Recently we were changing some external connections from user/pass logon to SNC logon without password and we had a problem which were really difficult to find out. So for future reference if anyone face the same problem this post might be helpful.
User on SAP
Here you can see the user has password disable and can be accessed using SNC
But when we tried this connection we were getting the error: CALL_FUNCTION_SIGNON_INCOMPL The logon data for user "" is incomplete
And the external application were getting the error JCO_ERROR_LOGON_FAILURE: Initialization of repository destination.....failed: Logon data incomplete.
To solve this we had to delete the entry related to the SNC name on the user on table SNCSYSACL, transaction SNC0