Upgrade from 3.0.12 to 3.1.1 caused a 404

I just upgraded my Komento 3.0.12 to 3.1.1 (downloaded the ZIP, and uploaded it via Manage -> Install). The installation worked but when I press the "Continue with Installation »" I get an joomla 404... "404 Component not found. "

When I reload my side, everything is gone ... and I think also my comments...