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Have you tried following the troubleshooter instructions for this error message? Please read the first two paragraphs of that page. You this doesn't help, you have a broken or partial backup archive.

Sorry, I just realised it's installing Joomla!, not a backup archive. Eek, I got confused among all those tickets. The correct documentation page is https://www.akeebabackup.com/documentation/admin-tools/updating-joomla.html and you should read "What happens if I get a message about a truncated or corrupt archive?" on that page. Sorry for the confusion.

Just in case the problem is with the PHP cURL extension, I'd recommend using Joomla!'s integrated updater (Components, Joomla! Upgrade). That component is also written by yours truly, uses similar code to Admin Tools with a few major differences:

• The update source is the official Joomla! XML stream. Admin Tools uses a page scrapping method to detect the latest version published in joomlacode.org.
• The download uses the core Joomla! API. Admin Tools uses its own downloader which uses cURL.

If the Joomla! Update component fails too you'll have to do a manual update. I won't give these instructions unless it's absolutely necessary. Let's try using Joomla! Update first.