ERROR validateIpnContent: VMPAYMENT_PAYPAL_ERROR_POSTING_IPN

[optmax]

VM 2.0.26d
J! 2.5.16

I had two system emails with same timestamp warning of a PP payment error.

In the VM PP log there was s single entry:
ERROR validateIpnContent: VMPAYMENT_PAYPAL_ERROR_POSTING_IPN

On my PP account looking at the IPN History I had processed a partial refund at about that time.  The refund was not registered in the backend (I may have had the order page open) but there was no error at the PP end - HTTP response code 200 - so the server received the IPN.

I resent the PP IPN and it was processed properly in the backend.

Any ideas as to the cause of the error?