This message may be encountered under a few different circumstances.
The most common will immediately follow the application having been started or restarted. Even though all the containers may show they are running, ALM is still starting up within its container and the application is not ready yet. This message will clear and display the ALM login page once the application has fully started. Typically less than 5 minutes.
If the message persists for more than 10 minutes it is very likely that the oracle DB connection string on the settings tab of the control panel has been misconfigured or some other network related issue is preventing ALM from connecting to the database.