Power Query refresh error

I built a small app which uses two Power Queries (pulling security prices from the Yahoo Finance website). The query results are loaded into a PQ table.

When a command button is clicked the two queries are refreshed using the following code:

    Dim q As WorkbookQuery
    For Each q In ThisWorkbook.Queries
        With ThisWorkbook.Connections("Query - " & q.Name).OLEDBConnection
            .BackgroundQuery = False
            .Refresh
        End With
    Next q
    Application.CalculateUntilAsyncQueriesDone

My problem is that this code sometimes executes error free and sometimes results in:

    Run time error 1004

    Applicaiton-defined or Object-defined error

The error occurs on the line ".BackgroundQuery = False" (in red above).

Does somebody know why this code sometimes results in error 1004? 

Thanks in advance for any suggestions.

Roland