Magento 2.1.9 Throwing 501 When User is Logged in And Calling /rest/default/V1/carts/mine/selected-payment-method

Question

I'm having an issue with my 2.1.9 Magento site where a guest user can checkout with PayPal without any issue but when a logged in user attempts to they're unable to.

During the checkout process there is a call to /rest/default/V1/carts/mine/selected-payment-method after you select PayPal as the payment method you wish to use.

The data sent to the server in this call is:

{"cartId":"10","method":{"method":"paypal_express","po_number":null,"additional_data":null}}

The response is:

{"method":null}

The call also returns a 501 which leads to an exception being thrown in the JS.

The JS that powers this is:

define(
    [
        'jquery',
        'Magento_Checkout/js/model/quote',
        'Magento_Checkout/js/model/url-builder',
        'mage/storage',
        'Magento_Checkout/js/model/error-processor',
        'Magento_Customer/js/model/customer',
        'Magento_Checkout/js/model/full-screen-loader'
    ],
    function ($, quote, urlBuilder, storage, errorProcessor, customer, fullScreenLoader) {
        'use strict';

        return function (messageContainer) {
            var serviceUrl,
                payload,
                method = 'put',
                paymentData = quote.paymentMethod();

            /**
             * Checkout for guest and registered customer.
             */
            if (!customer.isLoggedIn()) {
                serviceUrl = urlBuilder.createUrl('/guest-carts/:cartId/set-payment-information', {
                    cartId: quote.getQuoteId()
                });
                payload = {
                    cartId: quote.getQuoteId(),
                    email: quote.guestEmail,
                    paymentMethod: paymentData
                };
                method = 'post';
            } else {
                serviceUrl = urlBuilder.createUrl('/carts/mine/selected-payment-method', {});
                payload = {
                    cartId: quote.getQuoteId(),
                    method: paymentData
                };
            }
            fullScreenLoader.startLoader();

            return storage[method](
                serviceUrl, JSON.stringify(payload)
            ).fail(
                function (response) {
                    errorProcessor.process(response, messageContainer);
                    fullScreenLoader.stopLoader();
                }
            );
        };
    }
);

Answer 1

Found the issue, Magento had nothing to do with it.

The problem was caused by an Apache config issue.

For reference please see this post.