It simply means that your client application was previously not correctly configured i.e. the jndi.properties file is missing or incorrect.
While clustering was working, everything was fine in your case because, as part of clustering, automatic discovery of the replicated JNDI tree is performed when the configuration in your jndi.properties file contains no correct node information.
When you have removed the cluster-service.xml, your client wasn't able to find a target node anymore.
=> configure jndi.properties correctly and everything will work.