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java.util.ConcurrentModificationException: null	at java.util.HashMap$HashIterator.remove(HashMap.java:811)	at java.util.AbstractCollection.remove(AbstractCollection.java:252)	at org.codehaus.plexus.util.xml.Xpp3Dom.removeChild(Xpp3Dom.java:262)	at org.sonatype.nexus.configuration.model.AbstractXpp3DomExternalConfigurationHolder.setCollection(AbstractXpp3DomExternalConfigurationHolder.java:228)	at org.sonatype.nexus.proxy.repository.AbstractGroupRepositoryConfiguration.clearMemberRepositoryIds(AbstractGroupRepositoryConfiguration.java:56)	at org.sonatype.nexus.proxy.repository.AbstractGroupRepository.setMemberRepositoryIds(AbstractGroupRepository.java:390)	at org.sonatype.nexus.rest.groups.AbstractRepositoryGroupPlexusResource.updateRepositoryGroup(AbstractRepositoryGroupPlexusResource.java:163)	at org.sonatype.nexus.rest.groups.AbstractRepositoryGroupPlexusResource.createOrUpdateRepositoryGroup(AbstractRepositoryGroupPlexusResource.java:139)	at org.sonatype.nexus.rest.groups.RepositoryGroupPlexusResource.put(RepositoryGroupPlexusResource.java:164)	at org.sonatype.plexus.rest.resource.RestletResource.storeRepresentation(RestletResource.java:327)	at org.sonatype.nexus.rest.NexusRestletResource.storeRepresentation(NexusRestletResource.java:95)	at org.restlet.resource.Resource.put(Resource.java:706)