The idp.intercept.External.externalPath property defines the flow redirection path to the resource that's used to perform the external interceptor's logic, by default a context-relative location. It must be a resource with access to the container session. Modify as needed to match the location of your external interface (see the documentation on flow redirects).
On upgraded systems, the legacy file conf/intercept/external-intercept-config.xml may be removed if the property is defined in place of the original bean.
You may also dynamically derive the path to use, typically so that it can vary based on aspects of the request, by defining a bean (e.g., in global.xml) named shibboleth.intercept.externalPathStrategy of type Function<ProfileRequestContext,String>