It seems there is a minor bug in the CC.add method. Despite the fact that you can target a specific location to add a CC, if another CC is selected, the method fails:

Sub CCAddBug()
Dim oCC As ContentControl
Dim oRng As Word.Range
'Attempts to insert a CC as an optional range target fails if another CC has the focus (is selected)
  'Create three empty paragraphs.
  ActiveDocument.Range.Text = vbCr + vbCr
  Set oRng = ActiveDocument.Range(0, 0)
  'Add CC at oRng.
  Set oCC = ActiveDocument.ContentControls.Add(wdContentControlText, oRng)
  'Notice oCC.Range is now selected.
  Set oRng = ActiveDocument.Paragraphs(2).Range
  On Error GoTo err_Debugger
  'Since the previous CC is selected an error occurs.
  Set oCC = ActiveDocument.ContentControls.Add(wdContentControlText, oRng)
lbl_Exit:
  Exit Sub
err_Debugger:
  ActiveDocument.Paragraphs(3).Range.Select
  Resume
End Sub
Sub CCAddNoError()
Dim oCC As ContentControl
Dim oRng As Word.Range
  'Create three empty paragraphs.
  ActiveDocument.Range.Text = vbCr + vbCr
  Set oRng = ActiveDocument.Paragraphs(1).Range
  'Add CC at oRng.
  Set oCC = ActiveDocument.ContentControls.Add(wdContentControlText, oRng)
  'Notice oCC.Range is "NOT" selected.
  Set oRng = ActiveDocument.Paragraphs(2).Range
  'Since the previous CC is "NOT" selected there is no error.
  Set oCC = ActiveDocument.ContentControls.Add(wdContentControlText, oRng)
lbl_Exit:
  Exit Sub
End Sub