You have to deactive all third party plugins before Upgrading your Elgg install. Try to get on your public_html and delete all the non-default elgg plugins...
or you can lgin via ftp/cpanel and create a file called "disabled" inside the mod directory. This will disable all plugins. Now try to login and disable all the plugins required. After that you can remove the "disabled" file
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Security issues should be reported to security@elgg.org!
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