FWIW, errors like this typically happens when latin1 or cp1252 chars are passed to python routines that are expecting utf-8 encodings. In this case, an error is being detected or generated by the plugin and it is the error message itself that is causing the issue as its contents are not utf-8.
You could run epubcheck via the command line and see what gets sent to std error to see what might be going on. To prevent this issue in future versions of the launcher I will tell it to 'replace' on error.
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