Sharepoint Search not working - Start address sts4://servername/contentdbid cannot be crawled

I've spent a lot of time with sharepoint and when it works it works well but we keep having search issues with one of our sharepoint sites.

In the eventlog and sharepoint log files I get the following error:

The start address sts4://servername/contentdbid={66e8aaa2-3335-40fb-a679-41731a41d83b} cannot be crawled.

Context: Application 'Serve_search_queries_over_help_content', Catalog 'Search'

Details:
The object was not found. (0x80041201)

There are lots of posts about disabling loopback checking, and rebuilding search indexes around but my scenario is slightly different and didn't fall into these categories, although I DID disable loopback checking as part of the work towards the fix so that MAY BE REQUIRED.

I tried browsing to the URL that it mentioned and it didn't work. I didn't expect it to but I thought maybe its searching an old sharepoint site so concentrated on trying to find out how to rebuild the index. I couldn't find anything.

So then there was some mention of Alternative Access mappings and thats where I found the solution.

Our server is running Sharepoint Foundation Server 2010

It also hosts a number of other sites. We're using host headers to point users to the right site.

We're using a different URL (the host header) for accessing the sharepoint site.

So when I browse to http://servername I get nothing - to be expected. Thats when it hit me, its trying to access the site using the wrong URL.

What had happened is that for some other reason we'd changed the default URL in the Alternative Access Mapping to be http://servername and set an internet URL of kb.abc.com

What I did to fix the issue was set the default to http://kb.abc.com (this is the host header used for the site) and set the intranet URL to http://servername (although this wont work because its not in the host headers - I guess I could have added that too).