We had TeamCity 2020.2.4 running on Windows Server 2012 R2. A reverse proxy server using IIS has been setup. IIS handles the certificates. The database is SQL Server. Creating new accounts have been turned off and all accounts are authorized with TeamCity SAML authentication plug-in.
On the website I scheduled an update to 2021.1.4. The update, according to what I see in the logs, went well. The server started up and is waiting for confirmation for the Schema update page. However, the server is not accepting requests and just returns "The service is unavailable.". The service is running and the teamcity-server.log states that there is a data upgrade required and gives the authentication token.
Why would the server be unavailable? If I reproduce the upgrade from 2020.2.4 to 2021.1.4 on a separate machine as a test, I do get similar logs. I also can see the webpage with the information about the data migration, the option to backup the database and a button to upgrade.