Difficult to say without seeing the mobi in question. There's a ton of stuff that can go wrong with those older mobis--they're often full of some amazing crud.
You can't post the mobi here if it's copyrighted, though.
You could try the standalone KindleUnpack tool and see if it has better luck. It won't automatically create an epub from a mobi, but it would give you the html at least.
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