Logged-in Customer Cannot Add Products to Cart

Question

We're currently on an instance of Magento 2.2.7 (most products and customers have been migrated from Magento 1).

We have a strange issue where a small number of our customers are trying to order some stuff but once they login to their account they are experiencing problems.

If they're not logged in products can be added to the cart but when they try to check out and login with "My Account" the products disappear.

Furthermore if they try to add any products while logged in they cannot get them added to the cart.

They claim that they can still checkout as guests.

I cannot replicate the error myself and as this is only happening to a small number of customers I'm not sure what could be causing it or even how to test for it.

I wondered if anyone else has encountered this problem and if they found a cause or solution.

Answer 1

After some further Investigation I found the following error in the Apache Logs

2019-07-20 19:56:36 Error 54.171.125.80 AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to a member function getId() on null in /var/www/vhosts/mywebsite.com/httpdocs/app/code/Mirasvit/Rewards/Service/Customer/Tier.php:169\nStack trace:\n#0 /var/www/vhosts/mywebsite.com/httpdocs/app/code/Mirasvit/Rewards/Plugin/Balance/ChangePointsBalance.php(48): Mirasvit\Rewards\Service\Customer\Tier->updateCustomerTier('894')\n#1 /var/www/vhosts/mywebsite.com/httpdocs/vendor/magento/framework/Interception/Interceptor.php(135): Mirasvit\Rewards\Plugin\Balance\ChangePointsBalance->aroundChangePointsBalance(Object(Mirasvit\Rewards\Helper\Balance\Interceptor), Object(Closure), '894', '259', 'Earned 259 Rewa...', 'order_earn-1266...', true)\n#2 /var/www/vhosts/mywebsite.com/httpdocs/vendor/magento/framework/Interception/Interceptor.php(153): Mirasvit\Rewards\Helper\Balance\Interceptor->Magento\Framework\Interception\{closure}('894', '259', 'Earned 259 Rewa...', 'order_earn-1266...', true)\n#3 /var/www/vhosts/mywebsite.com/httpdocs/generated/code/Mirasvit...\n', referer: https://www.mywebsite.com/checkout/

I got in touch with Mirasvit and they provided a new version of the extension. With any luck this will resolve the issue.