I have this error sometimes when there is a reference to a footnote or something missing. A link to an footnote, where the file which contains the footnotes is not specified.
However, looking at the error, I think there is an another error in place. There is a reference to the file 'tired -but-not-sleeping.epub', where I think you mean 'tired-but-not-sleeping.epub'. I think you have a space somewhere in a link.
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