iPhone, iPad, and some Android checkout spinning wheel of death

Reported Problem:

It is a mixed bag of iPhone, iPad, and a few Android devices that get to the checkout out screen and after entering name and address, selecting shipping, and clicking the Proceed to payment" button the screen goes to a faded white the loading gif just spins and nothing loads. I had one customer try it on an iPhone 10, iPhone X, iMac, and iPad with no success. A few customers have said that if they log out of the customer account they can checkout as a guest without issue.

Solution:

X-Cart support was able to see where the problem is with our iPhone and iPad checkout not loading. "There is a bug related to the undefined is not an object (evaluating 'this.input.commonController') error. It appears only on Fast Lane Checkout + Safari + iFrame. They are working on the permanent patch. The following is what they suggest for the time being: 

As a temporary workaround I suggest one of the following options: 

1) Turn off the iFrame 

2) Switch to One Page Checkout mode 

3) Add one more payment method that would be loaded first on the checkout page so that the iFrame only displayed after the customer selects the credit card method.