Error 500 when searching for repository

application trhrows exception and blocks the GUI:

2013-11-11 00:43:31 ERROR [t-1-thread-5453] - org.sonatype.nexus.events.IndexerManagerEventInspector -
Could not maintain index!
java.lang.NullPointerException: null
at org.apache.maven.index.context.DefaultIndexingContext.acquireIndexSearcher(DefaultIndexingC
ontext.java:498) ~[indexer-core-5.1.1-d928049c927da992071a6c6bba892568dbcf27a3.jar:5.1.1-d928049c927da
992071a6c6bba892568dbcf27a3]
at org.apache.maven.index.DefaultIndexerEngine.getOldDocument(DefaultIndexerEngine.java:156) ~
[indexer-core-5.1.1-d928049c927da992071a6c6bba892568dbcf27a3.jar:5.1.1-d928049c927da992071a6c6bba89256
8dbcf27a3]
at org.apache.maven.index.DefaultIndexerEngine.update(DefaultIndexerEngine.java:77) ~[indexer-
core-5.1.1-d928049c927da992071a6c6bba892568dbcf27a3.jar:5.1.1-d928049c927da992071a6c6bba892568dbcf27a3
]
at org.apache.maven.index.DefaultIndexer.addArtifactsToIndex(DefaultIndexer.java:110) ~[indexe
r-core-5.1.1-d928049c927da992071a6c6bba892568dbcf27a3.jar:5.1.1-d928049c927da992071a6c6bba892568dbcf27
a3]
at org.apache.maven.index.DefaultNexusIndexer.addArtifactToIndex(DefaultNexusIndexer.java:338)
~[indexer-core-5.1.1-d928049c927da992071a6c6bba892568dbcf27a3.jar:5.1.1-d928049c927da992071a6c6bba892
568dbcf27a3]
at org.sonatype.nexus.index.DefaultIndexerManager.addItemToIndex(DefaultIndexerManager.java:77
5) ~[nexus-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.index.DefaultIndexerManager.access$500(DefaultIndexerManager.java:186) ~
[nexus-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.index.DefaultIndexerManager$4.run(DefaultIndexerManager.java:719) ~[nexu
s-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.index.DefaultIndexerManager.sharedSingle(DefaultIndexerManager.java:2418
) ~[nexus-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.index.DefaultIndexerManager.addItemToIndex(DefaultIndexerManager.java:71
3) ~[nexus-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.events.IndexerManagerEventInspector.inspectForIndexerManager(IndexerMana
gerEventInspector.java:75) [nexus-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.events.IndexerManagerEventInspector.inspect(IndexerManagerEventInspector
.java:60) [nexus-indexer-lucene-plugin-2.3.1-01.jar:2.3.1-01]
at org.sonatype.nexus.events.DefaultEventInspectorHost$EventInspectorHandler.run(DefaultEventI
nspectorHost.java:187) [nexus-app-2.3.1-01.jar:2.3.1-01]
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895) [na:1.6
.0_43]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918) [na:1.6.0_4
3]
at java.lang.Thread.run(Thread.java:662) [na:1.6.0_43]