Upgraded from 12.0-U2.1 to 12.0-U3. Everything worked fine.
Server run fine for two days or so.
Today, I couldn't log in via WebUI. Worked with ssh. Tried "service nginx restart" but no luck. Then I did it the hard way -> "reboot".
As the server was up again, I could login. Discovered the alert and now I'm sending the debug files to help you get this nice piece of software improved.