Thank You nannid and Phil Lomas for your feedback. I believe I have now narrowed down the cause.

Can you please try the following fix and let me know if it works in your environments?
  1. Remove all products from the affected order (or cancel the order). Make sure the Order Total is Zero (the bug should cause this to be true).
  2. Make the code change listed below.
  3. Add the products back to your order (or to a new order).


Edit "/admin/includes/functions/extra_functions/edit_orders_functions.php" as follows:

Starting at line 1134 replace:
Code:
	// Remove order totals already present in the order
	$module_list = explode(';', (str_replace('.php', '', MODULE_ORDER_TOTAL_INSTALLED)));
	$order_totals = eo_get_order_total_by_order($oID);
	if($order_totals !== null) {
		foreach($order_totals as $class => $total) {
			$keys = array_keys($module_list, $class);
			foreach($keys as $key) {
				if($key != 'ot_local_sales_taxes') unset($module_list[$key]);
			}
		}
	}
With the following code:
Code:
	// Remove order totals already present in the order
	$module_list = explode(';', (str_replace('.php', '', MODULE_ORDER_TOTAL_INSTALLED)));
	$order_totals = eo_get_order_total_by_order($oID);
	if($order_totals !== null) {
		foreach($order_totals as $class => $total) {
			if($class == 'ot_local_sales_taxes') continue;
			$keys = array_keys($module_list, $class);
			foreach($keys as $key) {
				unset($module_list[$key]);
			}
		}
	}