The issue is common among merchants who have implemented custom solutions or just migrated to Magento 2 version. There are many variants of the problem outcome, though let’s consider the commonest.
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The solution is simple, there is every likelihood the problem resides in cache. So, enable all caching options built-in your Magento 2.
When adding products from the catalog or product page you are redirected to the checkout page with an empty cart.
The thing is that despite the first version, Magento 2 stores root URL in the third path, e.g.: design/head/includes. Usually, the issue occurs after domain change.
SET value = REPLACE(value, '<old root URL or domain>', '<new root URL or domain>')
WHERE path = 'design/head/includes';
rm -rf var/cache/*