I have a bit more information about what's causing this. Whenever I change the URL of a page, this error occurs.

I did a search and got this message:

The Search Index for the site is currently being built. Please wait a few minutes and try your search again.
 

After several minutes, the index folder is still empty.

Interestingly, deleting all the files in the index folder has the side effect of eliminating the errors whenever the URL of a page is changed. It's just that the search feature no longer works.

It turns out that the index process was failing because I created a subfolder in the index folder. Instead of deleting all the existing files, I moved them into the subfolder just in case I needed to move them back. After moving the subfolder and running another search, three files were created in the index folder: deletable, segments, _1p3.cfs.

However, I noticed countless errors in the log file. Here's the stack trace:

2008-08-28 11:54:54,292 [2884] ERROR mojoPortal.Business.WebHelpers.IndexWriterTask [(null)] - System.InvalidOperationException: There is an error in XML document (1, 2). ---> System.InvalidOperationException: <Envelope xmlns='http://schemas.xmlsoap.org/soap/envelope/'> was not expected.
at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderIndexItem.Read3_IndexItem()
--- End of inner exception stack trace ---
at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
at System.Xml.Serialization.XmlSerializer.Deserialize(TextReader textReader)
at mojoPortal.Business.SerializationHelper.DeserializeFromString(Type type, String serializedObject)
at mojoPortal.Business.WebHelpers.IndexWriterTask.ProcessQueue(DataTable q, String indexPath)

On top of that, whenever I run another search, I get the same exact error as I reported in the other thread about RSS feeds and the extjs-viewport1 skin. This happens when SearchResults.aspx is trying to render.

Yup. This worked. Thanks!