It might be the case that the plugin is not able to write the PHP session info (into the /var/lib/php/sessions directory ) because of permission issues, and hence it is failing at the authentication step.
Can you please make sure that the folder /var/lib/php/sessions has sufficient write permissions? You will need to assign the directory with the owner that has the appropriate write permissions.
You can contact your web hosting provider for the above if you are not sure about doing it yourself.
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