I'd strongly suggest opening a case with Tableau support while you wait to hear from other users here.
For me, this issue was resolved, but I'm not entirely sure how.
I went through the steps to generate and load a schema file for BI Connector. Somewhere I got the wrong IP/Port/DB Name which generated the above error, but because the schema was loaded into the BI Connector, it appeared to be connecting when it was not. Sometimes the best thing is to draw out your connections and walk through the steps again slowly.
Tableau ---> :27032 (PostgreSQL connection) BI Connector --> :27017 MongoDB/myDB
I don't know if this explanation will help someone, but I don't see any documentation for it anywhere. So maybe this will get someone on the right track if they encounter something similar.