We have a Magento 2.1.5 project. We are getting strange errors. At some moment the website works properly and the next day we get an error when we login to the admin. We get the following error:
Class Magento\Quote\Model\ValidationRules\QuoteValidationRuleInterface does not exist
We fixed this by deleting the vendor folder and then run composer update. The problem is that the error keeps coming back. Any thoughts?