When attempting to start an Agent either from command line or the Service Manager Dialog, the Agent will start briefly/write a log, but stops immediately with the following error:
U02000019 The Server answers from system 'PROD'. Disconnecting again.
The Agent's .ini file likely does not have the correct system name specified (it is set by default to UC4). This can be determined by opening the Agent's .ini file and checking the [GLOBAL] section:
Ensure that the Agent's .ini file has the SYSTEM= parameter filled out with the system name mentioned in the message (in this case, PROD).