Search Server 2010 - error on results page

Using Search Server 2010 Express, on WinServer 2008 R2.

I can access the search page via the IP address, for example:

http://10.41.70.55/advanced.aspx     (where xx substituted for actual IP).

A search will cause a results page to appear, but instead of results it displays the following error:

The Web application at http://xx.xx.70.55/ could not be found. Verify that you have typed the URL correctly. If the URL should be serving existing content, the system administrator may need to add a new request URL mapping to the intended application.

I've researched the error, and it seems I need to edit the Alternate Access Mappings, but I don't understand how that works or what I should do, since the IP address is clearly going to the correct address.

Are Alternate Access Mappings the solution here?  If the search server name is something like my-server.local-domain.net, how would this work?

Any help much appreciated!

Pete