Button from Ribbon UI should call VBA function

[Markus]

Hello All,
I have a button in Word via VBA that saves the Word document back to a SQL database.

Here is the button definition so far:

Set oCommandBar = CommandBars.Add(Name:="Exp", Position:=msoBarTop)
oCommandBar.Visible = True
   
Set oMenue = oCommandBar.Controls.Add(Type:=msoControlButton)
With oMenue
                .Caption = "Speichern nach Exp ..."
                .Style = msoButtonCaption 
                .OnAction = "SendToExp"
                .Visible = True
            End With

OnAction calls the function

Public Function SendToExp() As String
    Dim httpreq As MSXML2.XMLHTTP30
    Dim Data As String
    Dim prop As DocumentProperty
    Dim url As String
........
End Function

This has also worked flawlessly for years. Now I wanted to add a second button that also saves back a PDF.
Since I had to work on the macro anyway, I wanted to change the buttons to a modern ribbon UI.

New Ribbon UI XML

<customUI xmlns="Link office /2009/07/ customui">
    <ribbon startFromScratch="false">
  <tabs>
   <tab id="customTab" label="Justus Tab">
    <group id="customGroup" label="Justus Group">
     <button id="customButton" label="Exp1" imageMso="FileSave" size="normal" onAction="Messagebox" />
     <button id="customButton2" label="Exp2" imageMso="FileSave" size="normal" onAction="SendToExp" />
    </group>
   </tab>
  </tabs>
    </ribbon>
</customUI>

The button for the message box works:

Sub Messagebox(control As IRibbonControl)
   MsgBox("Hallo")
End Sub

When pressing the SendToExp button, an error occurs:

"Incorrect number of arguments or invalid assignment of a property".

Public Function SendToExp(control As IRibbonControl) As String
......
End Function

Can someone tell me how the call must be directed?

Best thanks and many greetings

Markus