I tried to stop a portlet and restart it. This was the error message I received:

DeployedAppli W WSVR0218W: No modules defined for application, YOUR_APPLICATION, on server, WebSphere_Portal, in cluster, WebSphereCluster
ApplicationMg W WSVR0100W: An error occurred initializing, YOUR_APPLICATION
com.ibm.ws.exception.ConfigurationWarning: No enabled modules defined for YOUR_APPLICATION at com.ibm.ws.runtime.component.DeployedApplicationImpl.initialize(DeployedApplicationImpl.java:437)
at com.ibm.ws.runtime.component.ApplicationMgrImpl.initializeApplication(ApplicationMgrImpl.java:158)
at com.ibm.ws.runtime.component.ApplicationMgrImpl.startApplication(ApplicationMgrImpl.java:470)
at java.lang.reflect.Method.invoke(Native Method)
at com.tivoli.jmx.modelmbean.MMBInvoker.invoke(MMBInvoker.java:46)
at com.tivoli.jmx.modelmbean.MMBInvoker.invokeOperation(MMBInvoker.java:115)
at com.tivoli.jmx.modelmbean.DynamicModelMBeanSupport.invoke(DynamicModelMBeanSupport.java:409)
at javax.management.modelmbean.RequiredModelMBean.invoke(RequiredModelMBean.java:323)
at com.tivoli.jmx.GenericMBeanSupport.invoke(GenericMBeanSupport.java:178)
at com.tivoli.jmx.MBeanAccess.invoke(MBeanAccess.java:113)
at com.tivoli.jmx.MBeanServerImpl.invoke(MBeanServerImpl.java:290)
at com.ibm.ws.management.AdminServiceImpl.invoke(AdminServiceImpl.java:658)
at com.ibm.ws.management.connector.AdminServiceDelegator.invoke(AdminServiceDelegator.java:130)
at java.lang.reflect.Method.invoke(Native Method)
at com.ibm.ws.management.connector.soap.SOAPConnector.invoke(SOAPConnector.java(Compiled Code))
at com.ibm.ws.management.connector.soap.SOAPConnector.service(SOAPConnector.java(Compiled Code))
at com.ibm.ws.management.connector.soap.SOAPConnection.handleRequest(SOAPConnection.java:55)
at com.ibm.ws.http.HttpConnection.readAndHandleRequest(HttpConnection.java:622)
at com.ibm.ws.http.HttpConnection.run(HttpConnection.java:447)
at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:681)

Solution was to enable ‘loading module to deployment target’ for both EAR and web application.
Select Enterprise Applications -> Your application -> target mappings -> click to target -> Check if enabled is set
go back to your application select web modules -> select your web application -> target mappings -> click to target -> Check if enabled is set

See also screenshots here: