The current version of Active Query Builder ASP.NET Edition saves its state in the ASP.NET session.
To use Active Query Builder in a web farm, you must set up separation by sessions so that all requests from a client with the same session (SessionID) would process the same web server.
Alternatively, you can configure a single session storage for the entire web farm. You can switch IIS to the custom session-state mode and use any custom session manager that implements IHttpSessionState interface. You can read this article to learn more about getting a session state server working with web farms.
You can use REDIS to cache session data. There's a particular component for this purpose: https://www.nuget.org/packages/Microsoft.Web.RedisSessionStateProvider/