Sylvain,
This may be a partial installation of your 4.2.0 version. Please upgrade to 4.2.1 to see if that solves the problem.
If you still have the issue, please download the Akeeba Backup file from our website and install it twice, back to back, without doing anything in between. The Joomla! Installer software sometimes "forgets" to copy some files during an upgrade. The "install twice" trick usually forces it to copy the missing files. Do not uninstall Backup, that will wipe out your configuration and possibly your stored backup archives.
Dale L. Brackin
Support Specialist
English: native
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
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My time zone is
EST (UTC -5) (click here to see my current time in Philadelphia, PA)