When this error occurs, the connector can still connect to the server and preview the data selected. The issue only occurs once an import is attempted.
If HTTPS has been enabled, it will look for a valid certificate when the data connector tries to load data. So the error could be caused by any of the following issues:
In order for the Intelligence Server to accept the import despite this error, you must add a line to the load_data.js file:

KB483290