We've been posting blog posts for this site for a while now on Umbraco and only now - starting last week- we're experiencing an error every time we try to publish new blog posts, we get the error below. How do we fix this so we can publish pages again? Thank you

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Server Error in '/' Application.

Value cannot be null. Parameter name: attribute

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentNullException: Value cannot be null. Parameter name: attribute

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[ArgumentNullException: Value cannot be null. Parameter name: attribute] System.Xml.Linq.XAttribute.opExplicit(XAttribute attribute) +88226 UmbracoExamine.UmbracoContentIndexer.ReIndexNode(XElement node, String type) +110 Examine.ExamineManager.ReIndexNode(XElement node, String type, IEnumerable`1 providers) +75 UmbracoExamine.UmbracoEventManager.content_AfterUpdateDocumentCache(Document sender, DocumentCacheEventArgs e) +173 umbraco.DocumentCacheEventHandler.Invoke(Document sender, DocumentCacheEventArgs e) +0 umbraco.content.FireAfterUpdateDocumentCache(Document sender, DocumentCacheEventArgs e) +24 umbraco.content.UpdateDocumentCache(Document d) +742 umbraco.content.UpdateDocumentCache(Int32 pageId) +52 umbraco.library.UpdateDocumentCache(Int32 DocumentId) +105 umbraco.cms.presentation.editContent.Publish(Object sender, EventArgs e) +371 System.EventHandler.Invoke(Object sender, EventArgs e) +0 umbraco.controls.ContentControl.savePublish(Object Sender, ImageClickEventArgs e) +46 System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +115 System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +124 System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10 System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13 System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +35 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1724