Hi Phil!
I managed to replicate the problem and it is caused by the bug. I have already reported it to the team and it will be fixed in next versions.
Meanwhile, you can apply a workaround fix. Change the classes/XLite/Module/XC/ProductVariants/Controller/Customer/ChangeAttributeValues.php file and replace there the following line:
PHP Code:
if ($variant && 0 < $variant->getAmount()) {
with the next one:
PHP Code:
if ($variant && 0 < $variant->getAvailableAmount()) {
After you rebuild the cache the problem will be fixed.
Tony.