Hmmmm . . . .
This is one of those problems that is kind of hard to diagnose from afar - my first suggestion is to take a look at the contents of the project in FCP and see if there is anything potentially problematic in making an XML.
I know that is vague but not sure what else to suggest at this point. I've moved some older FCP projects into Premiere via XML without problems.
I don't know how much effort you want to put into resolving this or creating a workaround, but you could try breaking the transfer up into smaller chunks and then, if successful, combining those smaller chunks in on project in Premire.
Try doing just a sequence or two and see if they make it across.
Also - is there another Premier Pro system that you could send the full project xml to and test there?
If you'd like to post the XML somewhere public, I'll download it and try to import /open it in PPro on my system.
MtD