I came across the same issue and the snapshot of my issue is below:
Few solutions i tried and it worked:
1. if you have new data sources added to your workbook, please check the connection is happening from tableau server (source) to the new data source server. In my case, the connection test failed and had to add the TNS name entry in TNSNAMES.ORA file (oracle DB) to the server. It worked then
2. Even after adding a TNS connection name entry into the TNSNAMES.ORA file doesn't work, check if you need to open a firewall between your server (tableau) and Data source DB server. Once firewall is open, it should work.