I've done some experiments with parameters of the SaveAs method and when I used this form
wb.SaveAs(FileName, Excel.XlFileFormat.xlWorkbookNormal, null, null, false, false, Excel.XlSaveAsAccessMode.xlNoChange, false, false, null, null, null);
I have access to VBProject so it seems that enumeration
Excel.XlSaveAsAccessMode.xlShared
is in this case responsible for my problem.