Incorrect Payment Processor for Recurring Payments
The issue arises due to there being two AuthNet payment processors, one for both A1 and A2. And the CiviCRM code is (naively) written assuming there'd be a maximum of one. So when AuthNet calls into CiviCRM to notify it of a successful recurring payment, for the A1 payments it is loading the A2 payment processor — and then failing.
The attached patch uses information from the address that AuthNet posts its updates to, which can signal which payment processor to use.