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The error suggests that there is broken XML in the content.opf of that particular epub--probably the first one in the merge.
You might be able to clear the problem by opening the epub in Calibre's Edit book, make some minor change (add a space to a file, then delete) and save it again. Or Edit book may point out the problem.
You could also try an epub validator.
Without seeing the epub myself, that's about all I can suggest.
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