@David Heffernan: Well, writing a short demonstrating program solved my problem.
The original program opened an *.mhtml file and tried to convert it to doc format. And there I had the problem. When you create a new doc you can save it it any format without problem. That lead me to think that maybe the issue was that I saved from a non-native format to another non-native format.
So, my solution was to save it twice: first in the native format and then to the old format:
procedure SaveDocFile(WordDoc: Variant; FileName: string);
const wdFormatDocumentDefault=16;
begin
WordDoc.ActiveWindow.View.Type := wdPrintView;
if WordDoc.Application.Version='14.0' then
begin
WordDoc.SaveAs2(FileName := FileName, FileFormat := wdFormatDocumentDefault);
WordDoc.SaveAs2(FileName := FileName, FileFormat := wdFormatDocument);
end
else
WordDoc.SaveAs(FileName := FileName, FileFormat := wdFormatDocument);
end;