This error means that a client request to the QueryBuilder server instance failed because the server did not find a proper instance for it. In the current version, a server instance must be explicitly initialized before a client can communicate with it.
In the case of a classic WebForms, MVC, or even a client-side rendering application hosted on a single server, an instance is created automatically on page load. However, in a loosely coupled environment, when static HTML content is returned separately, a client must first send a request to the server to initialize a QueryBuilder instance on the server.
The same situation may arise due to the session's expiration. So we've added the reinitUrl parameter to handle the session expiration issue.
So the first thing you should check on getting this error is that the QueryBuilder instance is actually created on the server. BTW, this may happen due to the CORS misconfiguration if you handle QueryBuilder requests on a different host.