I encountered this issue with one of our servers. It turned out it was the “debug display” that was enabled for WordPress. I didn’t dig any further into the issue but my guess is there is a warning/notice being appended to a file the Auth.net plugin is validating or causing something to load too late. The solution for me was to just disable debugging display in wp-config.php by setting the following.
define( 'WP_DEBUG_DISPLAY', FALSE );
If you still need the debugging information, the plugin will write to a log if you enable the setting from the payment configuration screen in wp-admin. You can access the logs from WooCommerce > Status on the Logs tab. You will need to select the file matching the current timestamp. It should be similar to authorize_net_cim_credit_card-*.log
