Hi,
The error message "Invalid document end" actually comes from Eway themselves because their server couldn't read the payment request sent to it by the eway payment plugin, which actually uses the EwayPaymentLib class helper provided by eway to format the request to their server.
So I would normally say that there is something that changed on eway that would require modifications in their class helper or the plugin. But if it works for some payments and not others, then I don't know.
Maybe it comes from the product name, or the address of the customer which uses special characters that eway doesn't like ?
Can you confirm whether that is the case or not ?
Between a payment that works and one that doesn't, what is the difference ? The products purchased ? The address ? What if you change to the other one ? Does that make it work ? In that case what is the text involved with the problem ?