Hi,
We are experiencing exactly the same problem...the failure appears to be around 10% of orders. We receive an email from Worldpay telling us the the purchase has been made but the user never reaches checkout_success.php, and there for the the osc admin does not wnow what the customer has pordered. This has been happening for some time now more than a year but has got worse in the last three months.
We have tried the solution above regarding the suhosin.get.max_value_length but it has not solved the issue.
Has anybody else experienced this issue or can offer a solution?
Thanks,
Nick.