[jjk]

First, thanks for your answers

I have the ebook "Quatre-vingt-Treize - Victor Hugo.epub" (but it is same with many other ebooks)

@DNSB :
When I open it as a zip file (with any dezipper) I see among others 'mimetype' file which contents 20 bytes : application/epub+zip
And my zipper (7zip) indicates that 'mimetype' has a 20 bytes lenght, meaning that it is not compressed.
More : If I open the ebook in a hex viewer, I see the "mimetypeapplication/epub+zip" string, so I am sure that this file is not compressed.

When I open the ebook in Calibre-editor I get next tree (in left panel) :
Texte : titlepage.xhtml, hugo1.xhtml,.., hugo5.xhmtl, all in root
Styles : stylesheet.css, in root
Images : cover.jpg, in root
Polices : (none)
Divers : content.opf, toc.ncx both in root, and container.xml in META-INF\

@Kovid
Scenario of the crash :
For information
When I click on container.xml, I see its content :

<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
   <rootfiles>
      <rootfile full-path="content.opf" media-type="application/oebps-package+xml"/>
   </rootfiles>
</container>

Now, I click on the unpack book tool in the toolbar.
Editor asks me to choose a directory. I click on an empty directory (of any drive), then on the 'Select a dir' button. That is the crash. I get a dialog box intituled ERROR : Error when opening the book. I click on the button to display details :
calibre, version 5.5.0
ERREUR : Erreur lors de l'ouverture du livre: Erreur lors de l'ouverture du livre. Cliquer sur le bouton "Afficher les détails" pour plus d'information.

Traceback (most recent call last):
  File "calibre\gui2\tweak_book\job.py", line 34, in run
  File "calibre\gui2\tweak_book\boss.py", line 83, in get_container
  File "calibre\ebooks\oeb\polish\container.py", line 1560, in get_container
  File "calibre\ebooks\oeb\polish\container.py", line 1207, in __init__
calibre.ebooks.oeb.polish.container.InvalidEpub: No META-INF/container.xml in epub

BTW, just after clicking OK on the error message, I try to open any file in the left panel. I get then another error :

calibre, version 5.5.0
ERREUR : Exception non gérée: <b>FileNotFoundError</b>:[Errno 2] No such file or directory: 'C:\\Users\\xyz\\AppData\\Local\\calibre-cache\\ee\\tmpgxor4ucf\\a\\00001-rx2mbj5l\\META-INF\\container.xml'

calibre 5.5 [64bit]  embedded-python: True is64bit: True
Windows-10-10.0.19041 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.19041')
Python 3.8.5
Windows: ('10', '10.0.19041', '', 'Multiprocessor Free')
Interface language: fr
Successfully initialized third party plugins: CalibreSpy (1, 0, 83) && Diaps Editing Toolbag (0, 3, 7) && Ex Libris (1, 0, 19) && Job Spy (1, 0, 188) && KindleUnpack - The Plugin (0, 82, 3) && Kobo Utilities (2, 11, 8) && Open With (1, 5, 13) && Prettify Cover (1, 5, 0)
Traceback (most recent call last):
  File "calibre\gui2\tweak_book\boss.py", line 1619, in edit_file_requested
  File "calibre\gui2\tweak_book\boss.py", line 1588, in edit_file
  File "calibre\ebooks\oeb\polish\container.py", line 614, in raw_data
  File "calibre\ebooks\oeb\polish\container.py", line 1070, in open
FileNotFoundError: [Errno 2] No such file or directory: 'C:\\Users\\xyz\\AppData\\Local\\calibre-cache\\ee\\tmpgxor4ucf\\a\\00001-rx2mbj5l\\META-INF\\container.xml'

As you can see, I have some plugins installed...
And, indeed, file is corrupted in memory.

[EDIT] After having uninstalled all those plugings, I have still the same errors.[/EDIT]