ADMU3028I may appear when attempting to start a JVM, Dmgr, or Node Agent. As the error suggests, there is some other process on the server using the port that the JVM, Dgmr, or Node Agent is configured to use. For example, lets say port 8080 is already in use.
On Linux, the lsof command can be used to determine the process associated with the port.
~]# lsof -i:8080 java 12345 root 301u IPv6 12345678 0t0 TCP example.com:8080 (ESTABLISHED)
In this example, PID 12345 already has port 8080. If you know this process should not be running, you can end the process.
~]# kill -9 12345
Once killed, you should then be able to start the JVM, Dmgr, or Node Agent.