[Leonatus]

As far as I can see, C: drive is indeed the boot drive.
This time, I tried another book, but the error Code is:

calibre, version 2.77.0 (win32, embedded-python: True)
Konvertierungsfehler: Fehlgeschlagen: Buch 1 von 1 (Curiosités judiciaires et historiques du moyen âge. Procès contre les animaux) konvertieren

Buch 1 von 1 (Curiosités judiciaires et historiques du moyen âge. Procès contre les animaux) konvertieren
C:\Program Files\Calibre2\pylib.zip\dateutil\parser.py:339: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
Resolved conversion options
calibre version: 2.77.0
{'asciiize': False,
 'author_sort': None,
 'authors': None,
 'base_font_size': 0.0,
 'book_producer': None,
 'change_justification': u'original',
 'chapter': u"//*[((name()='h1' or name()='h2') and re:test(., '\\s*((chapter|book|section|part)\\s+)|((prolog|prologue|epilogue)(\\s+|$))', 'i')) or @class = 'chapter']",
 'chapter_mark': u'pagebreak',
 'comments': None,
 'cover': u'C:\\CalibreData\\Temporary\\calibre_mczi4k\\ontlwf.jpeg',
 'debug_pipeline': None,
 'dehyphenate': True,
 'delete_blank_paragraphs': True,
 'disable_font_rescaling': False,
 'dont_split_on_page_breaks': False,
 'duplicate_links_in_toc': False,
 'embed_all_fonts': False,
 'embed_font_family': None,
 'enable_heuristics': False,
 'epub_flatten': False,
 'epub_inline_toc': False,
 'epub_toc_at_end': False,
 'expand_css': False,
 'extra_css': u'@page {margin: 5pt}',
 'extract_to': None,
 'filter_css': u'',
 'fix_indents': True,
 'flow_size': 260,
 'font_size_mapping': None,
 'format_scene_breaks': True,
 'html_unwrap_factor': 0.4,
 'input_encoding': None,
 'input_profile': <calibre.customize.profiles.InputProfile object at 0x05D57BD0>,
 'insert_blank_line': False,
 'insert_blank_line_size': 0.5,
 'insert_metadata': False,
 'isbn': None,
 'italicize_common_cases': True,
 'keep_ligatures': False,
 'kepub_clean_markup': True,
 'kepub_disable_hyphenation': False,
 'kepub_hyphenate': True,
 'kepub_replace_lang': True,
 'language': None,
 'level1_toc': None,
 'level2_toc': None,
 'level3_toc': None,
 'line_height': 0.0,
 'linearize_tables': False,
 'margin_bottom': 5.0,
 'margin_left': 5.0,
 'margin_right': 5.0,
 'margin_top': 5.0,
 'markup_chapter_headings': True,
 'max_toc_links': 50,
 'minimum_line_height': 120.0,
 'no_chapters_in_toc': False,
 'no_default_epub_cover': False,
 'no_inline_navbars': False,
 'no_svg_cover': False,
 'output_profile': <calibre.customize.profiles.KoboReaderOutput object at 0x05D57FF0>,
 'page_breaks_before': u'/',
 'prefer_metadata_cover': False,
 'preserve_cover_aspect_ratio': False,
 'pretty_print': True,
 'pubdate': None,
 'publisher': None,
 'rating': None,
 'read_metadata_from_opf': u'C:\\CalibreData\\Temporary\\calibre_mczi4k\\rccg_y.opf',
 'remove_fake_margins': True,
 'remove_first_image': False,
 'remove_paragraph_spacing': False,
 'remove_paragraph_spacing_indent_size': 1.5,
 'renumber_headings': True,
 'replace_scene_breaks': u'',
 'search_replace': '[]',
 'series': None,
 'series_index': None,
 'smarten_punctuation': False,
 'sr1_replace': None,
 'sr1_search': None,
 'sr2_replace': None,
 'sr2_search': None,
 'sr3_replace': None,
 'sr3_search': None,
 'start_reading_at': None,
 'subset_embedded_fonts': False,
 'tags': None,
 'timestamp': None,
 'title': None,
 'title_sort': None,
 'toc_filter': None,
 'toc_threshold': 6,
 'toc_title': None,
 'transform_css_rules': '[]',
 'unsmarten_punctuation': False,
 'unwrap_lines': True,
 'use_auto_toc': False,
 'verbose': 2}
InputFormatPlugin: EPUB Input running
on C:\CalibreData\Temporary\calibre_mczi4k\15aa_8.epub
Found HTML cover titlepage.xhtml
Parsing all content...
Parsing Curiosites_judiciaires_et_histo_split_004.html ...
Parsing Curiosites_judiciaires_et_histo_split_002.html ...
Parsing page_styles.css ...
Parsing Curiosites_judiciaires_et_histo_split_005.html ...
Parsing titlepage.xhtml ...
Parsing stylesheet.css ...
Parsing Curiosites_judiciaires_et_histo_split_000.html ...
Parsing Curiosites_judiciaires_et_histo_split_003.html ...
Parsing Curiosites_judiciaires_et_histo_split_001.html ...
Reading TOC from NCX...
Merging user specified metadata...
Detecting structure...
Flattening CSS and remapping font sizes...
Source base font size is 12.00000pt
Removing fake margins...
Found 6 items of level: div_1
Found 172 items of level: p_1
div_1  left margin stats: Counter()
div_1  right margin stats: Counter()
Negative text indent detected at level  p_1, ignoring this level
Cleaning up manifest...
Trimming unused files from manifest...
Trimming u'titlepage.xhtml' from manifest
Creating KePub Output...
Rescaling image from 1200x1600 to 512x684 cover1.jpeg
Splitting markup on page breaks and flow limits, if any...
	Looking for large trees in Curiosites_judiciaires_et_histo_split_004.html...
	No large trees found
	Looking for large trees in Curiosites_judiciaires_et_histo_split_002.html...
	No large trees found
	Looking for large trees in Curiosites_judiciaires_et_histo_split_005.html...
	No large trees found
	Looking for large trees in Curiosites_judiciaires_et_histo_split_000.html...
	No large trees found
	Looking for large trees in Curiosites_judiciaires_et_histo_split_003.html...
	No large trees found
	Looking for large trees in Curiosites_judiciaires_et_histo_split_001.html...
	No large trees found
The cover image has an id != "cover". Renaming to work around bug in Nook Color
Copying file 'C:\CalibreData\Temporary\calibre_mczi4k\gbt7qw_KePubOutputPluginInfo' to 'C:\CalibreData\Temporary\calibre_mczi4k\nyrfhk_epub_container' as 'plugininfo.kte'
{'hyphenate': True, 'extended_kepub_features': True, 'clean_markup': True, 'smarten_punctuation': False, 'no-hyphens': False, 'replace_lang': True}
KoboTouchExtended:common:modify_epub:Found cover image ID 'cover'
KoboTouchExtended:common:modify_epub:Setting cover-image property
Forcing cleanup for file Curiosites_judiciaires_et_histo_split_000.html
Forcing cleanup for file Curiosites_judiciaires_et_histo_split_001.html
Forcing cleanup for file Curiosites_judiciaires_et_histo_split_002.html
Forcing cleanup for file Curiosites_judiciaires_et_histo_split_003.html
Forcing cleanup for file Curiosites_judiciaires_et_histo_split_004.html
Forcing cleanup for file Curiosites_judiciaires_et_histo_split_005.html
Forcing cleanup for file titlepage.xhtml
Cleaning markup for file Curiosites_judiciaires_et_histo_split_000.html
Cleaning markup for file Curiosites_judiciaires_et_histo_split_001.html
Cleaning markup for file Curiosites_judiciaires_et_histo_split_002.html
Cleaning markup for file Curiosites_judiciaires_et_histo_split_003.html
Cleaning markup for file Curiosites_judiciaires_et_histo_split_004.html
Cleaning markup for file Curiosites_judiciaires_et_histo_split_005.html
Cleaning markup for file titlepage.xhtml
Copying file 'C:\CalibreData\Temporary\calibre_mczi4k\kepub_za5f0m_hyphenate' to 'C:\CalibreData\Temporary\calibre_mczi4k\nyrfhk_epub_container' as 'kte-css/hyphenation.css'
Adding reference to kte-css/hyphenation.css to file Curiosites_judiciaires_et_histo_split_000.html
C:\Program Files\Calibre2\pylib.zip\ntpath.py:532: UnicodeWarning: Unicode unequal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
Python function terminated unexpectedly
  All strings must be XML compatible: Unicode or ASCII, no NULL bytes or control characters (Error Code: 1)
Traceback (most recent call last):
  File "site.py", line 132, in main
  File "site.py", line 109, in run_entry_point
  File "site-packages\calibre\utils\ipc\worker.py", line 195, in main
  File "site-packages\calibre\gui2\convert\gui_conversion.py", line 33, in gui_convert_override
  File "site-packages\calibre\gui2\convert\gui_conversion.py", line 26, in gui_convert
  File "site-packages\calibre\ebooks\conversion\plumber.py", line 1226, in run
  File "calibre_plugins.kepubout.conversion.kepub_output", line 131, in convert
  File "calibre_plugins.kepubout.common", line 103, in modify_epub
  File "calibre_plugins.kepubout.container", line 178, in add_content_file_reference
  File "lxml.etree.pyx", line 1445, in lxml.etree._Element.makeelement (src\lxml\lxml.etree.c:49848)
  File "apihelpers.pxi", line 133, in lxml.etree._makeElement (src\lxml\lxml.etree.c:14869)
  File "apihelpers.pxi", line 121, in lxml.etree._makeElement (src\lxml\lxml.etree.c:14748)
  File "apihelpers.pxi", line 268, in lxml.etree._initNodeAttributes (src\lxml\lxml.etree.c:16296)
  File "apihelpers.pxi", line 290, in lxml.etree._addAttributeToNode (src\lxml\lxml.etree.c:16701)
  File "apihelpers.pxi", line 1395, in lxml.etree._utf8 (src\lxml\lxml.etree.c:26485)
ValueError: All strings must be XML compatible: Unicode or ASCII, no NULL bytes or control characters

I dare say, there is nothing erroneous in the environment variable, but the error is the same.