I want to use TAdvGridDropDown to display data and let the user change values in a cell-dropdown and directly inserted (only numbers).
When I do the following:

AdvGrid.Grid.FixedRows := 0;
AdvGrid.Grid.Rowcount := 4;

The grid still shows 1 fixed row and zero normal rows.

From where do you call this?
Can you try to call this from grid.OnGetEditorProp()?

I called it in the form create procedure.
Calling it in OnGridGetEditorProp didn't change anything.

also when putting this in the beforeDropDown procedure:

procedure TFNewMessreihe.MyDDGridBeforeDropDown(Sender: TObject);
MyDDGrid.Grid.FixedRows := 0;
MyDDGrid.Grid.RowCount := 4;

It shows 0

This shows the dropdown grid without fixed row:

procedure TForm1.AdvGridDropDown1DropDown(Sender: TObject;
  var acceptdrop: Boolean);
  AdvGridDropDown1.Grid.RowCount := 10;
  AdvGridDropDown1.Grid.FixedRowAlways := false;
  AdvGridDropDown1.Grid.FixedRows := 0;

When I take your code exactly as you wrote it, use it in a brand new AdvGridDropDown with no settings changed within the DropDown Event procedure, it drops down as expected, showing 3 cols and 10 rows with prefilled content. Then I click ouside the grid so the drop down hides and click a second time on the drop down button and then I get an exception "Fixed row count must be less than row count".
Tested several times.

We could trace & fix this. The next update will address this.

thank you, this error is gone.

Is it possible to set the text of the control (not dropped down) by code?
I want to let the user set 4 different parameters (one parameter per row) and set the caption in the combo part as a combination of the 4 parameters (i.e.: 100: 2 : 50 : 800).
Is that possible? I don't see a caption of text property.

Text of the control is set with AdvGridDropDown.Text: string;