Just for completeness, I've added @Conversational to my conversational beans.
Now I got:
10:45:44,162 WARN [Component] Exception calling stateful session bean default @Remove method: versionBearbeiten javax.ejb.EJBException: org.jboss.seam.NoConversationException: no long-running conversation for @Conversational bean: versionBearbeiten
Yes. Try using conversation-required in pages.xml instead. It's probably more what you are after (requires that the page is accessed inside a lrc).