I'm getting the same mistake when trying to convert from epub to pdf (my books are in epub, but to save to disk in a separate location, I save them in pdf. I've done this almost since I started using Calibre and never had a problem before. Now I get the same error as Wipneus (from what I could see, although I will put my error message.
Spoiler:
Convert book 2 of 2 (The Beguile and Devotion of a Black Heir)
Resolved conversion options
calibre version: 2.32.0
{'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|pr ologue|epilogue)(\\s+|$))', 'i')) or @class = 'chapter']",
'chapter_mark': u'pagebreak',
'comments': None,
'cover': u'C:\\Users\\Wenpip\\AppData\\Local\\Temp\\calibre _kwh0ei\\vpqmhb.jpeg',
'custom_size': None,
'debug_pipeline': None,
'dehyphenate': True,
'delete_blank_paragraphs': True,
'disable_font_rescaling': False,
'duplicate_links_in_toc': False,
'embed_all_fonts': False,
'embed_font_family': u'Georgia',
'enable_heuristics': False,
'expand_css': False,
'extra_css': None,
'filter_css': u'',
'fix_indents': True,
'font_size_mapping': None,
'format_scene_breaks': True,
'html_unwrap_factor': 0.4,
'input_encoding': None,
'input_profile': <calibre.customize.profiles.InputProfile object at 0x0000000004AFD358>,
'insert_blank_line': False,
'insert_blank_line_size': 0.5,
'insert_metadata': False,
'isbn': None,
'italicize_common_cases': True,
'keep_ligatures': False,
'language': None,
'level1_toc': None,
'level2_toc': None,
'level3_toc': None,
'line_height': 0.0,
'linearize_tables': False,
'margin_bottom': 5.0,
'margin_left': 6.0,
'margin_right': 8.0,
'margin_top': 5.0,
'markup_chapter_headings': True,
'max_toc_links': 50,
'minimum_line_height': 120.0,
'no_chapters_in_toc': False,
'no_inline_navbars': False,
'old_pdf_engine': False,
'output_profile': <calibre.customize.profiles.KoboReaderOutput object at 0x0000000004AFDA20>,
'override_profile_size': False,
'page_breaks_before': '/',
'paper_size': u'letter',
'pdf_add_toc': False,
'pdf_default_font_size': 14,
'pdf_footer_template': None,
'pdf_header_template': None,
'pdf_mark_links': False,
'pdf_mono_family': u'Courier New',
'pdf_mono_font_size': 16,
'pdf_page_numbers': False,
'pdf_sans_family': u'Aharoni',
'pdf_serif_family': u'Times New Roman',
'pdf_standard_font': u'serif',
'prefer_metadata_cover': False,
'preserve_cover_aspect_ratio': False,
'pretty_print': False,
'pubdate': None,
'publisher': None,
'rating': None,
'read_metadata_from_opf': u'C:\\Users\\Wenpip\\AppData\\Local\\Temp\\calibre _kwh0ei\\xt3mot.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,
'uncompressed_pdf': False,
'unit': u'inch',
'unsmarten_punctuation': False,
'unwrap_lines': True,
'use_auto_toc': False,
'verbose': 2}
InputFormatPlugin: EPUB Input running
on C:\Users\Wenpip\AppData\Local\Temp\calibre_kwh0ei\ nvrvkq.epub
Parsing all content...
Parsing OEBPS/stylesheet.css ...
Parsing OEBPS/file0001.xhtml ...
Parsing OEBPS/file0004.xhtml ...
Parsing OEBPS/file0009.xhtml ...
Parsing OEBPS/file0007.xhtml ...
Parsing OEBPS/file0002.xhtml ...
Parsing OEBPS/file0008.xhtml ...
Parsing OEBPS/file0005.xhtml ...
Parsing OEBPS/title_page.xhtml ...
Parsing OEBPS/file0006.xhtml ...
Parsing OEBPS/file0003.xhtml ...
Reading TOC from NCX...
Merging user specified metadata...
Detecting structure...
Flattening CSS and remapping font sizes...
Embedding font: Georgia
Property: Unknown Property name. [1:42: panose-1]
Property: Unknown Property name. [1:47: panose-1]
Property: Unknown Property name. [1:49: panose-1]
Property: Unknown Property name. [1:54: panose-1]
Source base font size is 12.00000pt
Removing fake margins...
Found 10 items of level: div_1
Found 28 items of level: div_3
Found 9 items of level: div_2
Found 2700 items of level: p_3
div_1 left margin stats: Counter({u'': 9})
div_1 right margin stats: Counter({u'': 9})
div_3 left margin stats: Counter({u'': 28})
div_3 right margin stats: Counter({u'': 28})
div_2 left margin stats: Counter({u'': 9})
div_2 right margin stats: Counter({u'': 9})
p_3 left margin stats: Counter({u'0': 2700})
p_3 right margin stats: Counter({u'0': 2700})
Cleaning up manifest...
Trimming unused files from manifest...
Creating PDF Output...
Converting input as a text based book...
Serializing oeb input to disk for processing...
Rendering pages to PDF...
Processing C:\Users\Wenpip\AppData\Local\Temp\calibre_kwh0ei\ pbhj_e_pdf_out\OEBPS\title_page.xhtml...
I've also tried to convert one I converted 2 days ago. The only change I've made is to apply the Calibre update (to v2.32) this morning.
ETA: I was just told that there was a windows update this morning as well, not sure if anything there would affect this.