The last few lines, was that from the googlecheckout/response_error.log on your server?
The error you report is often generated when spaces or other characters are before (or within) the opening XML declaration. But that code is generated by the mod and then the data parsed by Google.
Previous link to Merchants forum posts will provide a little insight.
Probably best if you post issue in the GC Developers Forum (PHP API category) to hopefully obtain the attention of a Google "engineer". Note they don't have a good record of responding to all Q's over there so you may need to assert yourself. Include a link back to your post(s) in this thread to give them reference and be sure to report the version of the mod you are running. And ask for ropu ;-)
Woody
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