"Error: The user billing information is invalid"

I am having trouble getting Authorize.net payments to work on my website supportruggers.com. All looks good until the “Proceed to Pay” button returns “Error: The user billing information is invalid”.

I have tried multiple working cards, confirmed that Authorize.net is out of Test Mode, disabled all unnecessary plugins, and double-checked the API Login ID, Transaction Key, and Silent Post URL are set up correctly. Still no luck.

Somebody else posted the same issue here: https://wpinvoicing.com/support/topic/the-user-billing-information-is-invalid/

It sounds like this is caused by having “Allow posting without logging in” turned on, which is necessary for our site. It looks like there was a fix posted but it is marked as Private. Please help!