Versuch mal Excel und Word zu trennen, also er kopieren und dann den Word Teil. Vielleicht hilft das. Mir ist nicht ganz klar warum du das in der Reihenfolge ausführst...

Ändert sich so etwas?

Option Explicit

Sub Copy2Word()
Application.ScreenUpdating = False
 
        Call CheckPath(s_Path)
 
    'If n_CellsCounter = 1 And n_Zellenposition = 1 Then
    If b_WordDone = False Then
        b_WordDone = True
        Set app_word = CreateObject("word.application").Documents.Add(Worksheets("Parameter").Range("PAR_Template").Text).Application
        With app_word.ActiveDocument
            'Eingabedaten in das Production Sheet übernehmen
            .CustomDocumentProperties("D") = frm_Insert.txt_D.Text
            .CustomDocumentProperties("N") = frm_Insert.cbo_N.Text
            .CustomDocumentProperties("P") = frm_Insert.cbo_O.Text
            .CustomDocumentProperties("A") = frm_Insert.cbo_A.Text
            .CustomDocumentProperties("S") = s_FileName
            .CustomDocumentProperties("T") = frm_Insert.cbo_T.Text
            .CustomDocumentProperties("Date") = Date
            .CustomDocumentProperties("E") = frm_Insert.cbo_E.Text
            .CustomDocumentProperties("Z") = frm_Insert.cbo_Z.Text
            .CustomDocumentProperties("Te") = frm_Insert.txt_Te.Text
            .CustomDocumentProperties("In") = frm_Insert.cbo_In.Text
            .CustomDocumentProperties("Ku") = frm_Insert.cbo_Ku.Text
            .CustomDocumentProperties("Po") = frm_Insert.cbo_Po.Text
            .CustomDocumentProperties("Dru") = frm_Insert.txt_Dru.Text
            .CustomDocumentProperties("Dr") = frm_Insert.txt_Dr.Text
             
            'Dokument zum ersten Mal abspeichern
            .SaveAs Filename:=s_Path & s_FileName, FileFormat:=wdFormatDocument
        End With
    End If
    
        Sheets("Diagramme").Select
        ActiveSheet.ChartObjects("Chart 1").Activate
        ActiveChart.CopyPicture Appearance:=xlScreen, Size:=xlScreen, Format:= _
        xlPicture
        With app_word.Selection
            'name als Überschrift einfügen, Diagramme kopieren und anhängen
            .Move unit:=wdStory, Count:=1
            .InsertBreak Type:=wdPageBreak
            .Style = .ActiveDocument.Styles("Überschrift 1")
            .TypeText Text:=frm_InsertSolarCell.txt_Serie.Text & "_" & frm_InsertSolarCell.cbo_Device.Text & "_" & s_Zellenposition
            .TypeParagraph
            .TypeText Text:=frm_InsertSolarCell.txt_Kommentar.Text

            .TypeParagraph
            '.Selection.MoveDown unit:=wdLine 'stattdessen Enter eintragen
            .Paste
        End With
        
        ActiveSheet.ChartObjects("Chart 2").Activate
        ActiveChart.CopyPicture Appearance:=xlScreen, Size:=xlScreen, Format:= _
        xlPicture
        With app_word.Selection
            .TypeParagraph
            '.Selection.MoveDown unit:=wdLine 'stattdessen Enter eintragen
            .Paste
        End With

        Sheets("So").Select
        Cells(n_LastRowNr, Range("SC_Filename").Column).Select
        ActiveSheet.Hyperlinks.Add Anchor:=Selection, Address:=s_Path & s_FileName & ".doc", TextToDisplay:= _
        s_FileName
        
'Application.ScreenUpdating = False
End Sub

Und guck mal bei deinem Freund, ob unter VBA/Extras/Verweise irgendwelche Bibilotheken "NICHT VORHANDEN" drinnen sind, wenn ja nimm da mal den Haken raus und versuchs nochmal...