[paddyrm]

I get the same error since upgrading to 5.0.1:

calibre, version 5.1.0
ERROR: Loading book failed: Failed to open the book at C:\Users\Paddy\Documents\Calibre Library\calibre\The Guardian and The Observer (1770)\The Guardian and The Observer - calibre.mobi. Click "Show details" for more info.

Failed to convert book: C:\Users\Paddy\Documents\Calibre Library\calibre\The Guardian and The Observer (1770)\The Guardian and The Observer - calibre.mobi with error:
InputFormatPlugin: MOBI Input running
on C:\Users\Paddy\Documents\Calibre Library\calibre\The Guardian and The Observer (1770)\The Guardian and The Observer - calibre.mobi
MOBI markup appears to contain random bytes. Stripping.
MOBI markup appears to contain random bytes. Stripping.
Failed to run pipe worker with command: from calibre.srv.render_book import viewer_main; viewer_main()
Traceback (most recent call last):
File "site-packages\calibre\ebooks\mobi\reader\mobi6.py", line 188, in extract_content
File "site-packages\lxml\html\__init__.py", line 875, in fromstring
File "site-packages\lxml\html\__init__.py", line 761, in document_fromstring
File "src/lxml/etree.pyx", line 3237, in lxml.etree.fromstring
File "src/lxml/parser.pxi", line 1896, in lxml.etree._parseMemoryDocument
File "src/lxml/parser.pxi", line 1777, in lxml.etree._parseDoc
File "src/lxml/parser.pxi", line 1082, in lxml.etree._BaseParser._parseUnicodeDoc
File "src/lxml/parser.pxi", line 615, in lxml.etree._ParserContext._handleParseResultDoc
File "src/lxml/parser.pxi", line 725, in lxml.etree._handleParseResult
File "src/lxml/parser.pxi", line 654, in lxml.etree._raiseParseError
File "<string>", line 1
lxml.etree.XMLSyntaxError: encoding not supported USC4 little endian, line 1, column 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "site-packages\calibre\ebooks\conversion\plugins\mobi_in put.py", line 33, in convert
File "site-packages\calibre\ebooks\mobi\reader\mobi6.py", line 195, in extract_content
File "site-packages\lxml\html\__init__.py", line 875, in fromstring
File "site-packages\lxml\html\__init__.py", line 761, in document_fromstring
File "src/lxml/etree.pyx", line 3237, in lxml.etree.fromstring
File "src/lxml/parser.pxi", line 1896, in lxml.etree._parseMemoryDocument
File "src/lxml/parser.pxi", line 1777, in lxml.etree._parseDoc
File "src/lxml/parser.pxi", line 1082, in lxml.etree._BaseParser._parseUnicodeDoc
File "src/lxml/parser.pxi", line 615, in lxml.etree._ParserContext._handleParseResultDoc
File "src/lxml/parser.pxi", line 725, in lxml.etree._handleParseResult
File "src/lxml/parser.pxi", line 654, in lxml.etree._raiseParseError
File "<string>", line 1
lxml.etree.XMLSyntaxError: encoding not supported USC4 little endian, line 1, column 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "site-packages\calibre\ebooks\mobi\reader\mobi6.py", line 188, in extract_content
File "site-packages\lxml\html\__init__.py", line 875, in fromstring
File "site-packages\lxml\html\__init__.py", line 761, in document_fromstring
File "src/lxml/etree.pyx", line 3237, in lxml.etree.fromstring
File "src/lxml/parser.pxi", line 1896, in lxml.etree._parseMemoryDocument
File "src/lxml/parser.pxi", line 1777, in lxml.etree._parseDoc
File "src/lxml/parser.pxi", line 1082, in lxml.etree._BaseParser._parseUnicodeDoc
File "src/lxml/parser.pxi", line 615, in lxml.etree._ParserContext._handleParseResultDoc
File "src/lxml/parser.pxi", line 725, in lxml.etree._handleParseResult
File "src/lxml/parser.pxi", line 654, in lxml.etree._raiseParseError
File "<string>", line 1
lxml.etree.XMLSyntaxError: encoding not supported USC4 little endian, line 1, column 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "runpy.py", line 194, in _run_module_as_main
File "runpy.py", line 87, in _run_code
File "site.py", line 89, in <module>
File "site.py", line 84, in main
File "site.py", line 55, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 195, in main
File "<string>", line 1, in <module>
File "site-packages\calibre\srv\render_book.py", line 871, in viewer_main
File "site-packages\calibre\srv\render_book.py", line 862, in render_for_viewer
File "site-packages\calibre\srv\render_book.py", line 839, in render
File "site-packages\calibre\ebooks\oeb\iterator\book.py", line 63, in extract_book
File "site-packages\calibre\customize\conversion.py", line 245, in __call__
File "site-packages\calibre\ebooks\conversion\plugins\mobi_in put.py", line 39, in convert
File "site-packages\calibre\ebooks\mobi\reader\mobi6.py", line 195, in extract_content
File "site-packages\lxml\html\__init__.py", line 875, in fromstring
File "site-packages\lxml\html\__init__.py", line 761, in document_fromstring
File "src/lxml/etree.pyx", line 3237, in lxml.etree.fromstring
File "src/lxml/parser.pxi", line 1896, in lxml.etree._parseMemoryDocument
File "src/lxml/parser.pxi", line 1777, in lxml.etree._parseDoc
File "src/lxml/parser.pxi", line 1082, in lxml.etree._BaseParser._parseUnicodeDoc
File "src/lxml/parser.pxi", line 615, in lxml.etree._ParserContext._handleParseResultDoc
File "src/lxml/parser.pxi", line 725, in lxml.etree._handleParseResult
File "src/lxml/parser.pxi", line 654, in lxml.etree._raiseParseError
File "<string>", line 1
lxml.etree.XMLSyntaxError: encoding not supported USC4 little endian, line 1, column 1