I created an account just to be able to say thanks for posting this - I was actually trying to resolve the same issue on a Wordpress site, but your solution gave me the info I needed to figure it out - saving me a lot of time and frustration for sure. Thank you so much for posting a follow up with the resolution - I appreciate you!
edit: more info here for anyone that happens to find this via Google like I did
https://community.developer.authorize.net/t5/Integration-and-Testing/SSL-Certificate-Problem-Another-Authorize-net-Outage/td-p/62246