Error Struts

[MercedesH]

Saludos...

Tengo el siguiente problema en mi aplicación:

11:18:16,828 ERROR [Engine] StandardWrapperValve[action]: Servlet.service() for servlet action threw exception
java.lang.IllegalArgumentException: Path PrincipalDef does not start with a "/" character

struts-config:

Código HTML:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts-config PUBLIC "-//Apache Software Foundation//DTD Struts Configuration 1.1//EN" "http://jakarta.apache.org/struts/dtds/struts-config_1_1.dtd">
<struts-config>
  <form-beans>
    <form-bean name="loginForm" type="com.enelbar.seguridad.struts.LoginForm" />
    <form-bean name="principalForm" type="com.enelbar.seguridad.struts.PrincipalForm" />
  </form-beans>
  <global-forwards>
    <forward name="cerrarsesion" path="/CerrarSesion.jsp" />
  </global-forwards>
    <action-mappings>
    <action input="/Login.jsp" name="loginForm" path="/Login" scope="request" type="com.enelbar.seguridad.struts.LoginAction" validate="false">
      <forward name="mostrar" path="/Login.jsp" />
      <forward name="mostrarprincipal" path="PrincipalDef" />
    </action>
    <action input="/Principal.jsp" name="principalForm" path="/Principal" scope="request" type="com.enelbar.seguridad.struts.PrincipalAction" validate="false">
      <forward name="mostrarprincipal" path="PrincipalDef" />
    </action>
  </action-mappings>
</struts-config> 

tiles-defs:

Código HTML:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE tiles-definitions PUBLIC "-//Apache Software Foundation//DTD Tiles Configuration 1.1//EN" "http://jakarta.apache.org/struts/dtds/tiles-config_1_1.dtd">
<tiles-definitions>
  <definition name="index" path="/layout.jsp">
    <put name="title" value="SIAR" />
    <put name="header" value="/header.jsp" />
    <put name="nombrePagina" value=" " />
    <put name="leftmenu" value="/leftmenu.jsp" />
    <put name="body" value="/index.jsp" />
    <put name="footer" value="/footer.jsp" />
  </definition>
  
  <definition name="tituloHeader" path="/header.jsp">
    <put name="nombreMostrar" value="&gt;&gt; Seguridad" />
  </definition>
  <definition extends="index" name="PrincipalDef" path="/layout.jsp">
    <put name="nombrePagina" value="Principal" />
    <put name="body" value="/Principal.jsp" />
    <put name="header" value="tituloHeader" />
  </definition>
</tiles-definitions> 

En el action de Login:

Código:

.
.
.
return actionMapping.findForward(mostrarprincipal);

Por favor agradeceria enormemente su ayuda, he revisado de todo y nada q logro resolver.