Steps to reproduce

1. go to https://demoauthor.magnolia-cms.com/.magnolia/admincentral#app:configuration:browser;/modules/multisite/config/sites/fallback/domains@extends:treeview: Enter "../../fallback" 
2. go to https://demoauthor.magnolia-cms.com/.magnolia/admincentral#app:configuration:browser;/modules/multisite/config/sites/fallback/mappings@extends:treeview:  Enter "../../fallback" 

Magnolia crashes with a 

SEVERE: Servlet.service() for servlet [default] in context with path [/magnoliaAuthorDev] threw exception
java.lang.ClassCastException: class info.magnolia.cms.i18n.DefaultI18nContentSupport cannot be cast to class info.magnolia.cms.beans.config.URI2RepositoryMapping (info.magnolia.cms.i18n.DefaultI18nContentSupport and info.magnolia.cms.beans.config.URI2RepositoryMapping are in unnamed module of loader org.apache.catalina.loader.ParallelWebappClassLoader

and can not be restarted again.