I have nothing to fix because I don't know what the problem is yet. Your error is coming from Authorize.net themselves. They're rejecting the XML. Have you tried contacting Authorize.net?

We have not touched Authorize.net payment gateway. There's no reason for it to spontaneously stop working other than issues outside of our control, which typically means Authorize.net changed something on their end.

You're welcome to PM (see link in signature) backend login credentials and will take a look, but you really need to contact Authorize.net and have them check their logs to see why it was rejected.

Have invested further and confirmed a bug with Guzzle >= 6. We recently updated to Guzzle 7. XML POST requests are not being sent correctly.

forge.joomlapolis.com/issues/8960

Was able to implement a fix. Once reviewed and merged a new CBSubs release will be available fixing this issue. Have confirmed using Authorize.net sandbox the fix is successful and recurring profiles are creating.

Latest CBSubs build release should be available now with this fixed.

Was there any notifications in CBSubs > Notifications at the time of renewal? Silent Post should be logged there if it was sent by Authorize.net and received by CBSubs. If there's no notifications there then be sure the URL is absolutely correct and contact Authorize.net to see if they sent the silent post to that URL successfully. Accessing the URL directly isn't going to work as it's meant to receive payment data so direct access by you is not going to confirm anything. If your server or site has any kind of firewalls be sure they allow authorize.net IP Addresses through (you'll need to get those from them).