After upgrading from Plesk 11 to 12, its possible to get a 404 page not found error the first time you try to visit the admin panel of Plesk.

A server reconfiguration will give you this warning:

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nginx: [warn] conflicting server name "" on 0.0.0.0:8443, ignored

And the problem will remain.

A quick fix is to visit directly the address that apache listens and enter to the CP panel without SSL.

But to permanently fix this error you can do the following:

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rm /etc/sw-cp-server/conf.d/agent.conf
service sw-cp-server stop
service sw-cp-server start

Error fixed!