To help diagnose, comment out the error handler code (below). Then does the error message now have a debug button and if yes, what line is highlighted.

Note: you will have to re-enable Application.Events after the error.

Private Sub Worksheet_Change(ByVal Target As Range)
Dim oldVal As String
Dim newVal As String
Dim i As Long
Dim bRemoved As Boolean
Dim v As Variant

If Target.Count = 1 Then
    If Not Intersect(Target, Columns(2).SpecialCells(xlCellTypeAllValidation)) Is Nothing Then
        Application.EnableEvents = False
'        On Error GoTo ExitHandler
        If Len(Target.Value) Then
            newVal = Target.Value
            Application.Undo
            oldVal = Target.Value
            If oldVal <> "" Then
                v = Split(oldVal, ", ")
                oldVal = ""
                For i = LBound(v) To UBound(v)
                    If v(i) <> newVal Then
                        oldVal = oldVal & IIf(Len(oldVal), ", ", "") & v(i)
                    Else
                        bRemoved = True
                    End If
                Next i
                Target.Value = oldVal & IIf(bRemoved, "", ", " & newVal)
            Else
                Target.Value = newVal
            End If
        End If
    End If
End If
    
ExitHandler:
'  If Err.Number <> 0 Then MsgBox "Error " & Err.Number & vbLf & Err.Description, _
'                          vbCritical, "ERROR: Worksheet_Change procedure": Err.Clear
'  Application.EnableEvents = True
  
End Sub