Overview
By default agents use port 3839 as listening port for incoming connections. This error will pop up if the agent was not able to bind to the given port - it's being used by another process.
In most cases this error appears if two agents are installed on same computer - Management Console agent (installed by Management Console automatically) and another one installed here manually. 
To verify what other process uses the port:
- Windows: netstat -ano | findstr 3839 //see the last column for the process PID
- Linux: sudo netstat -ano | grep 3839
How to resolve this error?
Method 1) Leave only one agent on the computer, if the port is indeed occupied by an agent. You cannot uninstall Management Server agent though.
Method 2) Create new Agent profile for this agent only and change "Bind port" option there. Set it to 3840, for example. Use this profile for this agent.
Method 3) If the port is used by a third app, or your need two agents on this computer, set the agent to listen to a different port. Open Agent's config and add listening_port parameter there:
{ "listening_port": 3840,
"folders_storage_path": "%DOWNLOADS%",
"management_server": {
"host": "host:8444",
"cert_authority_fingerprint": "9...3a",
"bootstrap_token": "Y...U",
"disable_cert_check": false
}
Put the preferred port there and restart the agent.
Error code SE_BIND_PORT_BUSY