The "Revizto server is unreachable at the moment" error on the login screen of the Revizto app means that Revizto cannot connect to the server.
This usually happens if your company uses a proxy. In this case, you should open the Proxy Settings dialog and type in your company's proxy server address and login/password for it. Your IT should be able to provide these details. This will most likely be the quickest way to resolve the issue.
Another reason for this problem might be a firewall. Ask your IT to help you with that. See the Revizto network requirements for the information on how it should be set up.