Done. https://youtrack.jetbrains.com/issue/TW-56626.
The teamcity-server.log clearly show things going very wrong after the update. Around 2018-08-31 13:37:50 the errors start streaming in: “Unexpected response while executing internal Rest API request”.
The log file starts on 2018-08-24, but half of it is between 2018-08-31 13:37:50 and [2018-08-31 15:14:59, which is when we downgraded back to 2018.1.1.