Authorize.net Automatic Recurring Billing (ARB) has stopped working
All the payment processors have been tightening up security, especially with regards integrations between systems. 

My first inclination would be to check if the SSL postback that Authorize.net uses has the correct secure protocols set up.  Specifically, you should be sure that your server is configured to accept TLS 1.2 connections. Check with your hosting provider if necessary.

If this fails to help, I would recommend that you reach out to Authorize.net to see what information that may have in their logs with regards to the failure.