Strange 'Login Failed' in Crystal Reports.NET

I have my simple VB.net code thats got an external .rpt file (created using CR8.5) attached. I am using the LogonInfo for tables I use in my report. When I do the TestConnectivity() after ApplyLogonInfo() method, I invariably FALSE.
Actually my userid, password, servername, dbname are perfect. I use that to login to my db (using Sqlplus/Navigator).
How else I could narrow down my error? How can I check what else is happening in my program?
your inputs are highly appreciated.