When trying to connect to a data source in a second instance of Tableau Desktop (while another session is already running), one of the following error messages occurs:
An error occurred while communicating with Tableau Data Engine. Bad Connection: Tableau could not connect to the data source. There was problem and the data engine could not start properly.
There was a problem connecting to the "<data source name>" data source
In addition, Tableau Desktop may attempt for some time to connect to the data source with a window with the title "Processing Request" and the message, "Connecting to database..."
If Cancel is chosen, The message "Cancelling" may be displayed for several seconds before resuming the "Connecting to database..." message. This can happen repeatedly.