Working in a C/ALM scenario...
I created a test plan and test cases but then deleted them all and subsequently created new ones.
My use case in RRC remains linked to both the original (deleted) test case and the new one created via the requirements collection.

Message was "The link could not be deleted. See error log for details"

!ENTRY com.ibm.rdm.integration.calm 4 0 2010-01-13 11:13:16.781
!MESSAGE java.lang.reflect.InvocationTargetException
!STACK 0
java.lang.reflect.InvocationTargetException
at org.eclipse.jface.operation.ModalContext.run(ModalContext.java:421)
at org.eclipse.jface.window.ApplicationWindow$1.run(ApplicationWindow.java:758)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
at org.eclipse.jface.window.ApplicationWindow.run(ApplicationWindow.java:755)
at org.eclipse.ui.internal.WorkbenchWindow.run(WorkbenchWindow.java:2487)
at com.ibm.rdm.integration.calm.ui.actions.DeleteCalmLinkAction.run(DeleteCalmLinkAction.java:52)
at org.eclipse.jface.action.Action.runWithEvent(Action.java:498)
at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:583)
at org.eclipse.jface.action.ActionContributionItem.access$2(ActionContributionItem.java:500)
at org.eclipse.jface.action.ActionContributionItem$5.handleEvent(ActionContributionItem.java:411)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1003)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3823)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3422)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2384)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2348)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2200)
at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:495)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:288)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:490)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at com.ibm.rdm.ui.application.ReqComposerApplication.start(ReqComposerApplication.java:122)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:193)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:386)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
at java.lang.reflect.Method.invoke(Method.java:599)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:549)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:504)
at org.eclipse.equinox.launcher.Main.run(Main.java:1236)
at org.eclipse.equinox.launcher.Main.main(Main.java:1212)
Caused by: java.lang.RuntimeException: org.xml.sax.SAXParseException: The element type "HR" must be terminated by the matching end-tag "</HR>".
at com.ibm.rdm.fronting.server.common.xml.XMLParser.<init>(XMLParser.java:50)
at com.ibm.rdm.integration.calm.CalmLinkUtil.deleteBacklink(CalmLinkUtil.java:277)
at com.ibm.rdm.integration.calm.CalmLinkUtil.deleteBacklink(CalmLinkUtil.java:241)
at com.ibm.rdm.integration.calm.ui.actions.DeleteCalmLinkOperation.run(DeleteCalmLinkOperation.java:76)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:121)
Caused by: org.xml.sax.SAXParseException: The element type "HR" must be terminated by the matching end-tag "</HR>".
at org.apache.xerces.parsers.DOMParser.parse(Unknown Source)
at org.apache.xerces.jaxp.DocumentBuilderImpl.parse(Unknown Source)
at javax.xml.parsers.DocumentBuilder.parse(Unknown Source)
at com.ibm.rdm.fronting.server.common.xml.XMLParser.<init>(XMLParser.java:47)
... 4 more
Root exception:
java.lang.RuntimeException: org.xml.sax.SAXParseException: The element type "HR" must be terminated by the matching end-tag "</HR>".
at com.ibm.rdm.fronting.server.common.xml.XMLParser.<init>(XMLParser.java:50)
at com.ibm.rdm.integration.calm.CalmLinkUtil.deleteBacklink(CalmLinkUtil.java:277)
at com.ibm.rdm.integration.calm.CalmLinkUtil.deleteBacklink(CalmLinkUtil.java:241)
at com.ibm.rdm.integration.calm.ui.actions.DeleteCalmLinkOperation.run(DeleteCalmLinkOperation.java:76)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:121)
Caused by: org.xml.sax.SAXParseException: The element type "HR" must be terminated by the matching end-tag "</HR>".
at org.apache.xerces.parsers.DOMParser.parse(Unknown Source)
at org.apache.xerces.jaxp.DocumentBuilderImpl.parse(Unknown Source)
at javax.xml.parsers.DocumentBuilder.parse(Unknown Source)
at com.ibm.rdm.fronting.server.common.xml.XMLParser.<init>(XMLParser.java:47)
... 4 more