To confirm - you are checking on the computer where code errors?
Most likely problem is Microsoft Windows Common Controls-2 6.0 (SP4) reference.
Please try to change code to:
Private Sub UserForm_Initialize()
'Empty ClientCodeComboBox
Me.ClientCodeComboBox.Clear
'Fill ClientCodeComboBox
Me.ClientCodeComboBox.List = Worksheets("Index").Range("A2:A" & Worksheets("Index").UsedRange.Rows.Count).Value
'Prepare CompanyCodeBox.Caption
Me.ClientNameLabel.Caption = ""
'Uncheck CompanyCodeCheckBox
Me.CompanyCodeCheckBox.Value = False
'Clear YearEndDateBox
Me.YearEndDateBox.Value = VBA.Date
Me.YearEndDateBox.MaxDate = VBA.Date
'Clear ScanDateBox
Me.ScanDateBox.Value = VBA.Date
Me.ScanDateBox.MaxDate = VBA.Date
and test if there is still error.
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