Error: Query returned a result set.

I am connected to external database, and when I do a normal aggregate to return the data in a table, or an advanced query with a simple select query, I get this error:

and this from an aggregate:

I actually don't get the error!

Does anyone had this issue before?

My first guess would be an issue with the database connection, not the query itself.

I would suggest you re-do the integration, following the official guide step-by-step: