Using EE 2.5.3 and exp-resso 1.6.3
Basically we keep getting call-back failures. I'd guess around 5% of orders or maybe slightly less all encounter the exact same error:
Our systems have detected that your callback has failed. This callback failure means we were unable to pass information to your server about the following transaction: Transaction ID: Cart ID: xxxx Installation ID: xxxxxx Error reported: Callback to http://www.mysite.co.uk/index.php? ACT=66&H=381d0eea799cbf1ebbddbacca7718ee5: NOT OK, recevied HTTP status: 302 Server Reference: ukdc1-pz-pay03:callbackFailureEmail-1182:MerchReq-126-73
Note, it rarely contains a transaction ID which is very strange in and of itself.
World-Pay have insisted that everything is correct at their end and in the World-Pay settings. The only thing they weren't sure about was the call-back URL, which is set by the shopping cart module itself via it's call to WP.
Now, we know the server is working fine, we've had instances of successful payments very close either side of failed payments (as well as ones on their own) and Paypal is working fine with no callbacks ever.
If the server wasn't responding, it also would display a 5xx error, not the 302 redirect, which I believe is how the cart responds normally. We have a script that tests the server's response times from queries to the site and all have them are responding in the below 5 second range at worst.
So we pretty much can't think of anything else left to check.
Hope you can help!