SharePoint does not support reverse-proxy path modification

Using a load balancer to rewrite an incoming URL can be an attractive option for some situations (for example, where an incorrect URL has been widely advertised). However, it may not be a good idea for SharePoint sites.

From Technet http://technet.microsoft.com/en-us/library/cc288609.aspx:

Some reverse proxy devices can modify the path of a request (the portion of the URL that comes after the hostname and port number) in such a way that a request sent by the user to http://www.contoso.com/sharepoint/default.aspx, for example, is forwarded to the Web server as http://sharepoint.perimeter.example.com/default.aspx.

This is referred to as an asymmetrical path. Microsoft Office SharePoint Server 2007 does not support asymmetrical paths. The path of the URL must be symmetrical between the public URL and the internal URL. In the preceding example, this means that the "/sharepoint/default.aspx" portion of the URL must not be modified by the reverse proxy device.

Consider having the load balancer send a redirect response back to the browser instead of rewriting on the way to the server.