If the box is checked and the user clicks on it, it will be unchecked
before the
Click
event is raised.
So the first time you click, the box is unchecked, and your event handler does nothing. The second time you click, the box is checked, and your event handler unchecks it again.
Change your event handler to:
$_chkServer1.Add_Click({
if (-Not $_chkServer1.IsChecked)
{
$_chkServer1.Foreground = 'Green'
}
})