I ran Modify ePub on my test book with everything enabled. I ended up getting this log, although I'm not sure which option caused the error.
Code:
Modifying: C:\Users\[user]\AppData\Local\Temp\calibre_dpjv3o3j\bm1nikge_modify_epub\5036.epub
Updating metadata and cover
Parsing xml file: content.opf
Parsing xml file: toc.ncx
Looking for redundant entries in manifest
Looking for unmanifested files
Looking for NCX to flatten
No nested navPoints
Looking for broken links in the NCX
Looking for Adobe xpgt files and links to remove
Looking for pagemaps
Looking for Google Play pagemaps
Looking for Google Play pagemaps
Looking for Adobe DRM meta tags to remove
Looking for all jackets
Jacket removed: jacket.xhtml
Manifest item removed: jacket.xhtml (calibre_jacket)
Spine itemref removed: calibre_jacket
Adding or updating jacket
Manifest item added: jacket.xhtml (calibre_jacket)
Spine item inserted: calibre_jacket at pos: -1
Looking for html pages containing only broken image links
Looking for non dc: elements in manifest
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc6c0>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc8c0>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc9c0>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc940>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc840>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc600>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc780>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc680>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc740>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdce00>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcc80>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdca00>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdc440>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcdc0>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcd80>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcc00>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcb80>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcb00>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcec0>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcf00>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcbc0>
Removing child: <Element {http://www.idpf.org/2007/opf}meta at 0x16ff4cdcc40>
Looking for html files to remove charset meta tags/encode to utf-8
Switched to UTF-8 encoding for: text/part0000.html
Switched to UTF-8 encoding for: text/part0001.html
Switched to UTF-8 encoding for: text/part0002.html
Switched to UTF-8 encoding for: text/part0003.html
Switched to UTF-8 encoding for: text/part0004.html
Switched to UTF-8 encoding for: text/part0005.html
Switched to UTF-8 encoding for: text/part0006.html
Switched to UTF-8 encoding for: text/part0007.html
Switched to UTF-8 encoding for: text/part0008.html
Switched to UTF-8 encoding for: text/part0009.html
Switched to UTF-8 encoding for: text/part0010.html
Switched to UTF-8 encoding for: text/part0011.html
Switched to UTF-8 encoding for: text/part0012.html
Switched to UTF-8 encoding for: text/part0013.html
Switched to UTF-8 encoding for: text/part0014.html
Switched to UTF-8 encoding for: text/part0015.html
Switched to UTF-8 encoding for: text/part0016.html
Switched to UTF-8 encoding for: text/part0017.html
Switched to UTF-8 encoding for: text/part0018.html
Switched to UTF-8 encoding for: text/part0019.html
Switched to UTF-8 encoding for: text/part0020.html
Switched to UTF-8 encoding for: text/part0021.html
Switched to UTF-8 encoding for: text/part0022.html
Switched to UTF-8 encoding for: text/part0023.html
Switched to UTF-8 encoding for: text/part0024.html
Switched to UTF-8 encoding for: text/part0025.html
Switched to UTF-8 encoding for: text/part0026.html
Switched to UTF-8 encoding for: text/part0027.html
Switched to UTF-8 encoding for: text/part0028.html
Switched to UTF-8 encoding for: text/part0029.html
Switched to UTF-8 encoding for: text/part0030.html
Switched to UTF-8 encoding for: text/part0031.html
Switched to UTF-8 encoding for: text/part0032.html
Switched to UTF-8 encoding for: text/part0033.html
Switched to UTF-8 encoding for: text/part0034.html
Switched to UTF-8 encoding for: text/part0035.html
Switched to UTF-8 encoding for: text/part0036.html
Switched to UTF-8 encoding for: text/part0037.html
Switched to UTF-8 encoding for: text/part0038.html
Switched to UTF-8 encoding for: text/part0039.html
Switched to UTF-8 encoding for: text/part0040.html
Switched to UTF-8 encoding for: text/part0041.html
Switched to UTF-8 encoding for: text/titlepage.xhtml
Looking for embedded fonts
Looking for css @font-face style declarations
Looking for inline @font-face style declarations
Looking for CSS margins
Modified CSS margins in: page_styles.css
Modified CSS margins in: stylesheet.css
No Cats Allowed - ERROR: Traceback (most recent call last):
File "calibre_plugins.modify_epub.modify", line 79, in process_book
File "calibre_plugins.modify_epub.modify", line 175, in _process_book
File "calibre_plugins.modify_epub.modify", line 423, in _rewrite_css_margins
File "calibre_plugins.modify_epub.margins", line 170, in rewrite_css_margins
File "calibre_plugins.modify_epub.container", line 217, in get_raw
File "codecs.py", line 322, in decode
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte
ePub not changed after 0.70 seconds