To reproduce in magnolia.properties use info.magnolia.cms.security.SecuritySupport=info.magnolia.cms.security.RescueSecuritySupport

• login as superuser
• go to security app and change any user's property
• save

The saving operation is successful however the following warnings show up in the logs

WARN  agnolia.ui.workbench.tree.HierarchicalJcrContainer: Cannot determine parent for itemId: info.magnolia.ui.vaadin.integration.jcr.JcrNodeItemId@51e8122: javax.jcr.ItemNotFoundException: Root node doesn't have a parent
WARN  agnolia.ui.workbench.tree.HierarchicalJcrContainer: Cannot determine parent for itemId: info.magnolia.ui.vaadin.integration.jcr.JcrNodeItemId@d3a13629: javax.jcr.ItemNotFoundException: Root node doesn't have a parent

Did some debugging and the problem occurs when rendering the users view: info.magnolia.ui.workbench.tree.TreeRowScroller#expandTreeToNode tries to get the parent of the currently visible node. At some point, it gets to the root node and then the condition at #103 of info.magnolia.ui.workbench.tree.HierarchicalJcrContainer#getParent erroneously returns false. So instead of returning null the next instruction is executed which generates the warning (understandably, as root has no parent). Eventually the error is logged and null returned.