xtext: Xtext Nightly update site is broken

ERROR: org.eclipse.equinox.p2.director code=10053 Cannot complete the install because one or more required items could not be found.
  at org.eclipse.oomph.targlets.internal.core.TargletContainer.forceUpdate(TargletContainer.java:806)
  at org.eclipse.oomph.setup.targlets.impl.TargletTaskImpl$4.run(TargletTaskImpl.java:1167)
  at org.eclipse.oomph.util.pde.TargetPlatformUtil.runWithTargetPlatformService(TargetPlatformUtil.java:120)
  at org.eclipse.oomph.setup.targlets.impl.TargletTaskImpl.perform(TargletTaskImpl.java:1033)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.doPerformNeededSetupTasks(SetupTaskPerformer.java:3824)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.access$1(SetupTaskPerformer.java:3767)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer$WorkspaceUtil$1.run(SetupTaskPerformer.java:5125)
  at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2295)
  at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2322)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer$WorkspaceUtil.performNeededSetupTasks(SetupTaskPerformer.java:5119)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer$WorkspaceUtil.access$0(SetupTaskPerformer.java:5117)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.performNeededSetupTasks(SetupTaskPerformer.java:3758)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.performTriggeredSetupTasks(SetupTaskPerformer.java:3733)
  at org.eclipse.oomph.setup.internal.core.SetupTaskPerformer.perform(SetupTaskPerformer.java:3626)
  at org.eclipse.oomph.setup.ui.wizards.ProgressPage$9.run(ProgressPage.java:575)
  at org.eclipse.oomph.setup.ui.wizards.ProgressPage$11$1.run(ProgressPage.java:701)
  at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)
  ERROR: org.eclipse.equinox.p2.director code=0 Software being installed: artificial_root 1.0.0.v1559206377001
  ERROR: org.eclipse.equinox.p2.director code=0 Missing requirement: artificial_root 1.0.0.v1559206377001 requires 'org.eclipse.equinox.p2.iu; org.eclipse.xtext.xbase.lib.feature.group 2.14.0' but it could not be found

???