I've made a simple script that helped me pinpoint where the problem is located.
Maybe Lonkero can make more sense of it.
Basically the "$=Execute($Form.DoEvents())" causes the Fatal Exception, if you uncomment that line where thing works as it should but off course the loop keeps going and none of the events are triggered.
 Code:
Break On
$=SetOption('Explicit','On')
Dim $System, $Form, $Label1
Global $ComboBox1
$System = CreateObject("Kixtart.System")

$Form = $System.Form()
$Form.BackColor = 212,208,200
$Form.FontSize = 8,25
$Form.Height = 158
$Form.Left = 785
$Form.Text = "Test Dialog"
$Form.Top = 0
$Form.Width = 240

$Label1 = $Form.Controls.Label()
$Label1.BackColor = 212,208,200
$Label1.FontSize = 8,25
$Label1.Height = 16
$Label1.Left = 15
$Label1.Text = "User:"
$Label1.Top = 15
$Label1.Width = 59

$ComboBox1 = $Form.Controls.ComboBox()
$ComboBox1.DropDownWidth = 121
$ComboBox1.FontSize = 8,25
$ComboBox1.Height = 21
$ComboBox1.Left = 75
$ComboBox1.Sorted = "False"
$ComboBox1.Text = ""
$ComboBox1.Top = 15
$ComboBox1.Width = 121

$Form.Show
Init()
While $Form.Visible
  $=Execute($Form.DoEvents())
? @SERROR
Loop
Exit @ERROR

Function Init()
  Dim $Users, $User, $arrFilter[0]
  $arrFilter[0] = "User"
  $Users = GetObject("WinNT://@ldomain")
  $Users.filter = $arrFilter
  For Each $User In $Users
    $ComboBox1.AddItem ($User.Name)
  Next
EndFunction