\src\Core\Cart\CartRuleCalculator.php, function applyCartRule has the same line 2x. This line will generate a NAN if you have a free product via a cart rule in your basket, and another cart rule at the same time.
$discountAmountTaxExcl = $discountAmountTaxIncl / (1 + $taxRate)
The following code will work around the problem.
$discountAmountTaxExcl = ($discountAmountTaxIncl > 0) ?
$discountAmountTaxIncl / (1 + $taxRate) : 0;