Error 503 when Launching Enterprise Manager

I have recently faced an issue on a domain migration where, when accessing the Enterprise Manager console, the following error was shown: "Error 503-Service Unavailable".

In fact, after some investigation I noticed that the EM application was running and configured properly on the Weblogic.

By reading the Doc ID 1297193.1 on My Oracle Support I was able to figure out that the actual missing configuration was on the targets.xml file that holds the information of the Farm Domain the EM shall be targeting.

This targets.xml file is located under:
MW_HOME/user_projects/domains/<your_domain>/sysman/state

and has the following information details:
<Target TYPE="oracle_ias_farm" NAME="Farm_%DOMAIN-NAME%" DISPLAY_NAME="Farm_%DOMAIN-NAME%">
<Property NAME="MachineName" VALUE="%ADMINSERVER-HOST-NAME%"/>
<Property NAME="Port" VALUE="%ADMINSERVER-PORT%"/>
<Property NAME="Protocol" VALUE="%JMX-PROTOCOL%"/>
<Property NAME="isLocal" VALUE="true"/>
<Property NAME="serviceURL" VALUE="%SERVICE-URL%"/>
<Property NAME="WebLogicHome" VALUE="%WEBLOGIC-HOME%"/>
<Property NAME="DomainHome" VALUE="%DOMAIN-HOME%"/>
</Target>


On my case, this file was empty so I had to add the necessary information for the EM application to connect to my target Farm:
<Targets>
<Target TYPE="oracle_ias_farm" NAME="Farm_soa_domain" DISPLAY_NAME="Farm_soa_domain">
<Property NAME="MachineName" VALUE="myhostname.com"/>
<Property NAME="Port" VALUE="7001"/>
<Property NAME="isLocal" VALUE="true"/>
<Property NAME="Protocol" VALUE="t3"/>
<Property NAME="serviceURL" VALUE="service:jmx:t3://myhostname.com:7001/jndi/weblogic.management.mbeanservers.domainruntime"/>
<Property NAME="WebLogicHome" VALUE="/u01/app/oracle/Middleware/wlserver_10.3"/>
<Property NAME="DomainHome" VALUE="/u01/app/oracle/Middleware/user_projects/domains/soa_domain"/>
</Target>
</Targets>

More information on this issue on My Oracle Support.