Agent does not connect to the server
Either after initial setup or with "Agent has unregistered (will upgrade)" note in UI.
First, check the three Agents tab in UI ("Unauthorized", "Connected", "Disconnected") to check for the agent status visible by the server.
Wait for several minutes as agent connection can take time, especially in the slow network environments.
To investigate, check the agent logs for possible issues (teamcity-agent.log, launcher.log, upgrade.log).
If that does not bring up anything obvious and the agent environment complies with the requirements, please send us the full set of agent and server logs as well as screenshots of all the above mentioned Agents UI tabs.
Please refer to Common Problems