Umbraco Version: 4.5.0

We are using Examine but now for no reason I cannot publish any nodes - getting the error:-

Cannot index queue items, another indexing operation is currently in progress

[Exception: Indexing Error Occurred: Cannot index queue items, another indexing operation is currently in progress]
   Examine.LuceneEngine.Providers.LuceneIndexer.OnIndexingError(IndexingErrorEventArgs e) +239
   UmbracoExamine.UmbracoExamineIndexer.OnIndexingError(IndexingErrorEventArgs e) +388
   Examine.LuceneEngine.Providers.LuceneIndexer.ForceProcessQueueItems() +2551
   Examine.LuceneEngine.Providers.LuceneIndexer.SafelyProcessQueueItems() +167
   Examine.LuceneEngine.Providers.LuceneIndexer.AddSingleNodeToIndex(XElement node, String type) +546
   Examine.LuceneEngine.Providers.LuceneIndexer.ReIndexNode(XElement node, String type) +318
   Examine.ExamineManager._ReIndexNode(XElement node, String type, IEnumerable`1 providers) +206
   Examine.ExamineManager.ReIndexNode(XElement node, String type, IEnumerable`1 providers) +71
   UmbracoExamine.UmbracoEventManager.content_AfterUpdateDocumentCache(Document sender, DocumentCacheEventArgs e) +317
   umbraco.DocumentCacheEventHandler.Invoke(Document sender, DocumentCacheEventArgs e) +0
   umbraco.content.FireAfterUpdateDocumentCache(Document sender, DocumentCacheEventArgs e) +136
   umbraco.content.UpdateDocumentCache(Document d) +1680
   umbraco.content.UpdateDocumentCache(Int32 pageId) +120
   umbraco.library.UpdateDocumentCache(Int32 DocumentId) +196
   umbraco.cms.presentation.editContent.Publish(Object sender, EventArgs e) +785
   System.EventHandler.Invoke(Object sender, EventArgs e) +0
   umbraco.controls.ContentControl.savePublish(Object Sender, ImageClickEventArgs e) +165
   System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +134
   System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +165
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3691

Any ideas would be welcome.

Many Thanks

Aron