Problem:

• info.magnolia.cms.security.HierarchicalUserManager (magnolia-core) places the user accounts into a hierarchical node structure of the type ItemType.FOLDER.
• info.magnolia.cms.core.ItemType.NT_FOLDER = "nt:folder";
  -> "nt:folder" is not minxin:referencable
  -> therefore does not have a UUID
  -> therefore cannot be activated / deactivated
  -> therefore also produces an error message when being deleted

Solution:

• Changed ItemType.NT_FOLDER to "mgnl:folder"
• added this custom nodetype definition to custom_nodetypes.xml:
  <nodeType hasOrderableChildNodes="true" isMixin="false" name="mgnl:folder" primaryItemName=""><supertypes><supertype>mix:referenceable</supertype><supertype>nt:folder</supertype></supertypes><propertyDefinition autoCreated="false" mandatory="false" multiple="false" name="" onParentVersion="COPY" protected="false" requiredType="undefined"/><childNodeDefinition autoCreated="false" defaultPrimaryType="" mandatory="false" name="" onParentVersion="COPY" protected="false" sameNameSiblings="true"><requiredPrimaryTypes><requiredPrimaryType>nt:base</requiredPrimaryType></requiredPrimaryTypes></childNodeDefinition></nodeType>

Note:
I don't know where else nodes of this type are being used. Therefore I cannot say if this change affects anything else. For me everything has worked fine after this change.