This means that the temporary directory assigned to PHP (set up as /tmp) is not writable. SiteCenter relies on session variables to keep track of each connection. Usually, PHP saves the session variables on a file in the temporary directory.
If PHP cannot write the session file, it will not start the session. Without the session, you will not be able to login.
If you have this problem, contact your server administrator and ask him to set the permissions on your web server.

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