That's an odd error to see on that report - Tableau Server is just querying its own Postgres repository. Does this error always come up or just this once? Does it come up on any other reports? Has something changed recently? The answers to these questions will likely spawn more, but this is a starting place...
I don't think this has to do with connecting to the data source - something has just gone wrong. I bouncing the server and/or using tabadmin cleanup --restart will fix. If not, support