...
procedure TCustomFilterListView.SetFilterList(Value : TStringList);
var
  I : Integer;
begin
  FFilterList.Assign(Value);
  for I := 0 to FFilterList.Count - 1 do
    with Groups.Add do
      Header := TSelectedFilter(FFilterList.Objects[I]).Kategorie; // Läuft Fehlerfrei durch

end;
...
procedure TCustomFilterListView.FilterScan(Value : TFileScanRec);
var
  I : Integer;
  E : String;
  N : String;
  F : String;
  S : Cardinal;
begin
  E := ExtractFileExt(StrPas(Value.lpFileName));
  I := FFilterList.IndexOf(E);
  if ((I <> -1) and FFilterList.OwnsObjects) then
  begin
    N := FFilterList.Strings[I];
    F := TSelectedFilter(FFilterList.Objects[I]).FilterName; // <-- Hier der Fehler, keine Objekte vorhanden
    S := (Value.lpFindFileData.nFileSizeHigh * (Int64(MAXDWORD) + 1)) + Value.lpFindFileData.nFileSizeLow;
    FFoundFiles.AddObject(N, TFoundFile.Create(I, Value.lpFileName, S, F));
  end;
  if Assigned(FOnFilterScan) then FOnFilterScan(Self, StrPas(Value.lpFileName));
end;
...