Most likely some of the files in the epub are encoded as Windows-1252 files instead of UTF-8 files (in Windows-1252 encoded files, 0xe9 is the French character é).
Try the following:
1. Locate the Sigil plugin folder, by selecting Edit > Preferences > Open Preferences Location.
2. In the EpubCheck plugin folder, open plugin.py with a text editor.
3. Change the following lines (507 & 508):
Code:
stdout = result[0].decode(os_encoding)
stderr = result[1].decode(os_encoding)
to:
Code:
stdout = result[0].decode(os_encoding, "ignore")
stderr = result[1].decode(os_encoding, "ignore")
This change should allow you to run the plugin.