Resolution:
Note the following limitation of Designer 5.x.

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In Designer 5.x, a resource name must be non-null, containing no illegal characters (/ # \  : * ? " < > | , null character and any other illegal file name characters), not be duplicated in the group, not be a whole number enclosed in square brackets ([123]). A project name must not use the following characters: | / \ " ' : ?.

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Make sure that none of the processes and project names has these characters. Otherwise, the project will be able to be opened (worst case), or the buildear tool will fail. In both cases, the following error will be reported in the C:\Documents and Settings\<user name>\.TIBCO\logs\designer.log:

com.tibco.ae.designerapi.NameConflictException: Name contains illegal characters: ''
at com.tibco.ae.designerapi.DesignerResource.setName(DesignerResource.java:265)
at com.tibco.ae.tools.palettes.processpalette.ProcessDefinition.setModel(ProcessDefinition.java:1120)
at com.tibco.ae.tools.palettes.processpalette.ProcessDefinition.<init>(ProcessDefinition.java:233)
at com.tibco.ae.tools.palettes.processpalette.ProcessObjectFactory.readFromStream(ProcessObjectFactory.java:101)
at com.tibco.pe.load.ProcessModelXiNodeObjectFactory.newInstance(ProcessModelXiNodeObjectFactory.java:118)
at com.tibco.objectrepo.object.ObjectProvider.a(ObjectProvider.java:522)
at com.tibco.objectrepo.object.ObjectProvider.getObject(ObjectProvider.java:312)
at com.tibco.ae.designerapi.DesignerResourceStore.loadResourceFrom(DesignerResourceStore.java:735)
at com.tibco.ae.designerapi.DesignerResourceStore.loadResourcesFrom(DesignerResourceStore.java:699)
at com.tibco.ae.designerapi.DesignerResourceStore.initializeResource(DesignerResourceStore.java:1051)
at com.tibco.ae.designerapi.DesignerResourceStore.insureFoldersAreInitialized(DesignerResourceStore.java:1185)
at com.tibco.ae.designerapi.DesignerResourceStore.insureFoldersAreInitialized(DesignerResourceStore.java:1160)
at com.tibco.ae.tools.designer.designertools.ImportExportAction.<init>(Unknown Source)
at com.tibco.ae.tools.designer.designertools.ImportExport.getMenuItem(Unknown Source)
at com.tibco.ae.tools.designer.MenuManager.createToolsMenu(MenuManager.java:703)
at com.tibco.ae.tools.designer.MenuManager.init(MenuManager.java:104)
at com.tibco.ae.tools.designer.MenuManager.<init>(MenuManager.java:71)
at com.tibco.ae.tools.designer.AEWindow.initialize(AEWindow.java:177)
at com.tibco.ae.tools.designer.AEWindow.<init>(AEWindow.java:70)
at com.tibco.ae.tools.designer.AEDocument.initRootFolder(AEDocument.java:273)
at com.tibco.ae.tools.designer.AEDocument.<init>(AEDocument.java:131)
at com.tibco.ae.tools.designer.AEApplication.openDocument(AEApplication.java:1788)
at com.tibco.ae.tools.designer.AEApplication.openDocument(AEApplication.java:1701)
at com.tibco.ae.tools.designer.actions.OpenAction.performAction(OpenAction.java:36)
at com.tibco.ae.designerapi.actions.GenericDesignerAction.actionPerformed(GenericDesignerAction.java:300)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1849)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2169)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:420)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:258)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:5501)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3135)
at java.awt.Component.processEvent(Component.java:5266)
at java.awt.Container.processEvent(Container.java:1966)
at java.awt.Component.dispatchEventImpl(Component.java:3968)
at java.awt.Container.dispatchEventImpl(Container.java:2024)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4212)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3892)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3822)
at java.awt.Container.dispatchEventImpl(Container.java:2010)
at java.awt.Window.dispatchEventImpl(Window.java:1778)
at java.awt.Component.dispatchEvent(Component.java:3803)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)

Error, "com.tibco.ae.designerapi.NameConflictException: Name contains illegal characters: '' when trying to open a project using Designer, or buildear using the command line tool.