Intermittent "Start tag expected, '<' not found" errors with PxPay

[daneastmond]

We've been seeing some intermittent errors when calling PxPayCompleteAuthorizeRequest()

bad error simplexml_load_string(): Entity: line 1: parser error : Start tag expected, '<' not found in /omnipay/paymentexpress/src/Message/PxPayCompleteAuthorizeRequest.php

Been up and down the code and can't see any reason why this would be failing.

Perhaps a WindCave timeout?

Thought about wrapping the simplexml_load_string inside createResponse() in a try/catch block to provide a fallback, but a bit of a stab in the dark rather than understanding the problem.