I think this is an old issue that doesn't seem to be solved (it happened with the latest nightly).

An internal error occurred during: "'<local IndexJobManager>' - 0 exact matches, 0 potential matches. Total 0".
org.eclipse.core.internal.resources.ResourceException: Resource '/org.scala-refactoring.library' is not open.
	at org.eclipse.core.internal.resources.Project.checkAccessible(Project.java:150)
	at org.eclipse.core.internal.resources.Project.getReferencedProjects(Project.java:521)
	at org.scala.tools.eclipse.search.searching.ProjectFinder.refs(ProjectFinder.scala:36)
	at org.scala.tools.eclipse.search.searching.ProjectFinder.projectClosure(ProjectFinder.scala:25)
	at org.scala.tools.eclipse.search.searching.Finder.org$scala$tools$eclipse$search$searching$Finder$$relevantProjects(Finder.scala:172)
	at org.scala.tools.eclipse.search.searching.Finder$$anonfun$occurrencesOfEntityAt$2$$anonfun$apply$10$$anonfun$apply$11.apply(Finder.scala:138)
	at org.scala.tools.eclipse.search.searching.Finder$$anonfun$occurrencesOfEntityAt$2$$anonfun$apply$10$$anonfun$apply$11.apply(Finder.scala:136)
	at scala.Option.foreach(Option.scala:236)
	at org.scala.tools.eclipse.search.searching.Finder$$anonfun$occurrencesOfEntityAt$2$$anonfun$apply$10.apply(Finder.scala:136)
	at org.scala.tools.eclipse.search.searching.Finder$$anonfun$occurrencesOfEntityAt$2$$anonfun$apply$10.apply(Finder.scala:135)
	at scala.Option.foreach(Option.scala:236)
	at org.scala.tools.eclipse.search.searching.Finder$$anonfun$occurrencesOfEntityAt$2.apply(Finder.scala:135)
	at org.scala.tools.eclipse.search.searching.Finder$$anonfun$occurrencesOfEntityAt$2.apply(Finder.scala:132)
	at scala.tools.eclipse.ScalaPresentationCompiler.withSourceFile(ScalaPresentationCompiler.scala:113)
	at scala.tools.eclipse.ScalaProject$$anonfun$withSourceFile$1.apply(ScalaProject.scala:615)
	at scala.tools.eclipse.ScalaProject$$anonfun$withSourceFile$1.apply(ScalaProject.scala:614)
	at scala.tools.eclipse.ScalaProject$$anonfun$withPresentationCompiler$1.apply(ScalaProject.scala:608)
	at scala.tools.eclipse.ScalaProject$$anonfun$withPresentationCompiler$1.apply(ScalaProject.scala:607)
	at scala.tools.eclipse.util.Cached$class.apply(Cached.scala:41)
	at scala.tools.eclipse.ScalaProject$$anon$1.apply(ScalaProject.scala:113)
	at scala.tools.eclipse.ScalaProject.withPresentationCompiler(ScalaProject.scala:607)
	at scala.tools.eclipse.ScalaProject.withSourceFile(ScalaProject.scala:616)
	at scala.tools.eclipse.InteractiveCompilationUnit$class.withSourceFile(InteractiveCompilationUnit.scala:66)
	at scala.tools.eclipse.javaelements.ScalaSourceFile.withSourceFile(ScalaSourceFile.scala:60)
	at org.scala.tools.eclipse.search.searching.Finder.occurrencesOfEntityAt(Finder.scala:144)
	at org.scala.tools.eclipse.search.handlers.FindOccurrences$$anon$1.run(FindOccurrences.scala:75)
	at org.eclipse.search2.internal.ui.InternalSearchUI$InternalSearchJob.run(InternalSearchUI.java:91)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)