Cannot Go Past Billing Address During 1-Page Checkout

Version: 4.2
Checkout: 1-page checkout process

On a new store we're testing to go live, we have an issue when testing the checkout process where:
* Add an item to your cart
* Go to shopping cart
* Click the box for I agree to terms, and click Checkout
* The 1-page checkout page appears, with the first section for Billing Address
* Make sure an address is selected, click continue
* Page flashes, then returns to the main shopping cart page


Troubleshooting so far:
* No errors in the Admin area log
* Happens with different items in the cart
* Have enable/disable various payment providers, does not seem to make a difference

Work-around: If we switch to page-by-page checkout, it goes through each page correctly and completes the sale


Any ideas on what else to check for errors or troubleshooting? The page-by-page checkout works, but would prefer to have the 1-page checkout process available.

In Browser, turn on the developer tools (F12 in Chrome) and select the Console tab.  Step through checkout and see if any errors messages appear in the console.

Good call - was able to catch what it doesn't like:

Mixed Content: The page at 'https://www.domain.com/onepagecheckout' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://www.domain.com/checkout/OpcSaveBilling/'. This request has been blocked; the content must be served over HTTPS.

As mentioned in the error, the site is being served over HTTP with a certificate. However, it is going through CloudFlare CDN before it hits my actual nopCommerce host. Will that make an impact? Something about that configuration I forgot to setup properly?

Under Settings -> General -> Force SSL for all site pages = Checked
Under Settings -> Stores -> Store URL is using HTTPS

Edit: Possibly related. Checking F12 console on normal page loads, I'm getting:

Mixed Content: The page at 'https://www.domain.com/productName' was loaded over HTTPS, but requested an insecure image 'http://www.domain.com/images/thumbs/0000101_Logo_v2_WhiteOutline_TransparantBG.png'. This content should also be served over HTTPS.

So appears the pages themselves are loading on SSL, but the images and/or widgets may not be?

Configuration -> Store -> store -> SSL enabled, now setting it to true.

Seems to fix the problem, assuming I access everything without the www. on the URL. Will see about doing some DNS magic to take www. requests and redirect to the non-www version.

Guess we have to enable the SSL checkbox in both places for it to take hold on the whole site?