EDIT

When deleting a synonym (maybe also a taxon) an undefined exception is thrwon "Error thrown but no associated exception"

last remote method : http://api.cybertaxonomy.org:80/cichorieae/remoting/taxon.service
last remote request client time : 2021-06-07T21:56:43.4
last remote request response header time : Mon, 07 Jun 2021 19:56:43 GMT
client error time : 2021-06-07T21:56:43.432
login : admin
editor version : 5.24.0
server : api.cybertaxonomy.org (cybertaxonomy.org) / cichorieae
schema version : 5.23.0.0.20210422
os : Windows Server 2012 R2 6.3 amd64
java : 1.8.0_131
java.lang.Throwable: Error thrown but no associated exception
    at eu.etaxonomy.taxeditor.model.MessagingUtils.getDefaultThrowable(MessagingUtils.java:257)
    at eu.etaxonomy.taxeditor.model.MessagingUtils.errorDialog(MessagingUtils.java:327)
    at eu.etaxonomy.taxeditor.model.MessagingUtils.errorDialog(MessagingUtils.java:298)
    at eu.etaxonomy.taxeditor.bulkeditor.e4.handler.DeleteHandlerE4.delete(DeleteHandlerE4.java:279)
    at eu.etaxonomy.taxeditor.bulkeditor.e4.handler.DeleteHandlerE4.execute(DeleteHandlerE4.java:133)
    at sun.reflect.GeneratedMethodAccessor141.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:55)
    at org.eclipse.e4.core.internal.di.InjectorImpl.invokeUsingClass(InjectorImpl.java:282)
    at org.eclipse.e4.core.internal.di.InjectorImpl.invoke(InjectorImpl.java:264)
    at org.eclipse.e4.core.contexts.ContextInjectionFactory.invoke(ContextInjectionFactory.java:132)
    at org.eclipse.e4.core.commands.internal.HandlerServiceHandler.execute(HandlerServiceHandler.java:152)
    at org.eclipse.core.commands.Command.executeWithChecks(Command.java:494)
    at org.eclipse.core.commands.ParameterizedCommand.executeWithChecks(ParameterizedCommand.java:488)
    at org.eclipse.e4.core.commands.internal.HandlerServiceImpl.executeHandler(HandlerServiceImpl.java:210)
    at org.eclipse.e4.ui.workbench.renderers.swt.HandledContributionItem.executeItem(HandledContributionItem.java:433)
    at org.eclipse.e4.ui.workbench.renderers.swt.AbstractContributionItem.handleWidgetSelection(AbstractContributionItem.java:454)
    at org.eclipse.e4.ui.workbench.renderers.swt.AbstractContributionItem$3.handleEvent(AbstractContributionItem.java:482)
    at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
    at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4418)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1079)
    at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4236)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3824)
    at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$4.run(PartRenderingEngine.java:1121)
    at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
    at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1022)
    at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:150)
    at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:693)
    at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
    at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:610)
    at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:148)
    at eu.etaxonomy.taxeditor.Application.start(Application.java:20)
    at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
    at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
    at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
    at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:388)
    at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:243)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:673)
    at org.eclipse.equinox.launcher.Main.basicRun(Main.java:610)
    at org.eclipse.equinox.launcher.Main.run(Main.java:1519)

This makes it impossible to delete such corrupt records.