ERROR validateIpnContent: VMPAYMENT_PAYPAL_ERROR_POSTING_IPN

[optmax]

VM 2.6.6
J! 2.5.22
PHP 5.4.25

I had two system emails with same timestamp warning of a PP payment error and a link to te PayPal log.

(Similar thing to my  post several months ago http://forum.virtuemart.net/index.php?topic=122021.0)

This time...
Resending the IPN from the PP account generates the same error emails.

In the backend the order remains Pending - several other PP payments since have been processed correctly.

All seems to be OK at The PP end - the money is in my account and the HTTP response code for the IPN is 200

The error log has a lot more detail in this version.  What do I need to look at to diagnose the problem ?