SharePoint Search server getting gateway timeouts

I have a SharePoint 2010 installation with Search Server 2010 Express added on.  The server was working fine up until last month, when the SharePoint Server Search service Crawl Log began returning this error on all items on the site:

An unrecognized HTTP response was received when attempting to crawl this item. Verify whether the item can be accessed using your browser. ( Error from SharePoint site: HttpStatusCode GatewayTimeout The request failed with HTTP status 504: Unknown Host. )

Examining the SharePoint logs yield messages similar to the following, apparently 1 for each item being crawled (I've enabled Verbose mode on the Search service in hopes of getting more data):

58:52.9 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  Indexing      dm0c Verbose GetVirtualServerPolicy is about to call WS for virtual server, strVirtualServerUrl http://<REDACTED>

58:52.9 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  Indexing      dm01 Verbose GetWSSContent Calling ObjectType VirtualServer ObjectId folderUrl itemId retrieveChildItemsTrue securityOnly False lastItemIdOnPage 

58:52.9 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  Indexing      dm1q Verbose InjectClaimsCookies called for request url http://<REDACTED>/_vti_bin/sitedata.asmx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  Indexing      dm1k Verbose Exception Type: System.Net.WebException *** Message : The request failed with HTTP status 504: Unknown Host. *** StackTrace: at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)  at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)  at Microsoft.Office.Server.Search.Internal.Protocols.SiteData.SiteData.GetContent(ObjectType objectType, String objectId, String folderUrl, String itemId, Boolean retrieveChildItems, Boolean securityOnly, String& lastItemIdOnPage)  at Microsoft.Office.Server.Search.Internal.Protocols.SharePoint2006.SiteDataImpl.GetWSSContent(ObjectType objectType, String objectId, String folderUrl, String itemId... 

01/06/2011 00:58:53.10* mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  Indexing      dm1k Verbose ..., Boolean retrieveChildItems, Boolean securityOnly, String& lastItemIdOnPage)  at Microsoft.Office.Server.Search.Internal.Protocols.SharePoint2006.CSTS3Helper.GetVirtualServerPolicy(String strStsUrl, sPolicyUser[]& vUsers, sPolicyUser& anonymousUser, String& strVersion, String[]& vServerWFEs, Boolean& fIsHostHeader, Boolean& fIsAllUsersNT) 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  Indexing      fsa0 Monitorable GetVirtualServerPolicy fail. error 2147750404, strStsUrl http://<REDACTED> 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dvt6 High  SetSTSErrorInfo ErrorMessage = Error from SharePoint site: HttpStatusCode GatewayTimeout The request failed with HTTP status 504: Unknown Host. hr = 80041204 [sts3util.cxx:5095] d:\office\source\search\native\gather\protocols\sts3\sts3util.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dvi3 High  ***** Couldn't retrieve server http://<REDACTED> policy, hr = 80041204    [sts3util.cxx:1738] d:\office\source\search\native\gather\protocols\sts3\sts3util.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dvu0 High  STS3::StoreCachedError: Object initialization failed. Message: "Error from SharePoint site: HttpStatusCode GatewayTimeout The request failed with HTTP status 504: Unknown Host." HR: 80041204 [sts3util.cxx:5189] d:\office\source\search\native\gather\protocols\sts3\sts3util.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dvg4 High  ****** Server <REDACTED> security initialization failed, hr = 80041204 error Message Error from SharePoint site: HttpStatusCode GatewayTimeout The request failed with HTTP status 504: Unknown Host. [sts3util.cxx:1358] d:\office\source\search\native\gather\protocols\sts3\sts3util.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dv5x High  CSTS3Accessor::InitServer: Initialize STS Helper failed. Return error to caller, hr=80041204 [sts3acc.cxx:1691] d:\office\source\search\native\gather\protocols\sts3\sts3acc.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dv3t High  CSTS3Accessor::InitURLType fails, Url sts4://<REDACTED>/siteurl=/siteid={ef93c343-d8ff-4291-9cd6-46117f03c151}/weburl=p25/sr5/webid={2c1884b6-97d4-4d44-979e-dd54e4e3458d}/listid={b0ba2907-1a17-4c12-87cb-592d0e3feba2}, hr=80041204 [sts3acc.cxx:258] d:\office\source\search\native\gather\protocols\sts3\sts3acc.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dvb1 High  CSTS3Accessor::Init fails, Url sts4://<REDACTED>/siteurl=/siteid={ef93c343-d8ff-4291-9cd6-46117f03c151}/weburl=p25/sr5/webid={2c1884b6-97d4-4d44-979e-dd54e4e3458d}/listid={b0ba2907-1a17-4c12-87cb-592d0e3feba2}, hr=80041204 [sts3handler.cxx:312] d:\office\source\search\native\gather\protocols\sts3\sts3handler.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  PHSts       dvb2 High  CSTS3Handler::CreateAccessorExD: Return error to caller, hr=80041204   [sts3handler.cxx:330] d:\office\source\search\native\gather\protocols\sts3\sts3handler.cxx 

58:53.1 mssdmn.exe (0x2310)      0x24C4 SharePoint Server Search  FilterDaemon     e4ye High  FLTRDMN: Errorinfo is "Error from SharePoint site: HttpStatusCode GatewayTimeout The request failed with HTTP status 504: Unknown Host." [fltrsink.cxx:553] d:\office\source\search\native\mssdmn\fltrsink.cxx 

58:53.1 mssdmn.exe (0x2310)      0x2498 SharePoint Server Search  Indexing      dm0c Verbose GetVirtualServerPolicy is about to call WS for virtual server, strVirtualServerUrl http://<REDACTED> 

58:53.1 mssdmn.exe (0x2310)      0x2498 SharePoint Server Search  Indexing      dm01 Verbose GetWSSContent Calling ObjectType VirtualServer ObjectId folderUrl itemId retrieveChildItemsTrue securityOnly False lastItemIdOnPage 

A crawl (whether full or incremental) now takes an entire day; it used to take 2 hours on a full.

The IIS logs do not appear to show attempts to access the site (which does make sense, since we are getting issues resolving the site).  No Application/System errors are generated; only a warning:

The start address http://<REDACTED> cannot be crawled.

Context: Application 'Search_Service_Application', Catalog 'Portal_Content'

Details:

An unrecognized HTTP response was received when attempting to crawl this item. Verify whether the item can be accessed using your browser. (0x80041204)

While the farm account password was automatically changed by SharePoint, I do not believe this is the cause.  I've rebooted the server and all services/application pools returned without error.

I'm running out of ideas on what to do...help!