Crystal Reports: Login failed when trying to add database to report

[Michelle Stone]

In the crystal reporter.net designer I tried to add a
dataset to the "DATABASE FIELDS" node of the Field
Explorer tree. I get the following Messagebox:

Logon Failed
Details: Object reference not set to an instance of a
object

The message box title is "Crystal Reports ActiveX
Designer"

I tried adding other datasets in the project and they
work. What are the possible reasons that my dataset is
not working?

Thanks...