The user is being created in server but is not being updated in the redux store, it is getting rejected in the onQueryStarted function. Used RTK Query

I'm not entirely sure because there is very little to go off of but it looks like this is still a server side error. It seems like your code is able to save the user record, but right after that, something on the server side goes wrong? You need to provide more code and context

The server seems to return just a string message, not valid JSON.

In that case you will have to set responseHandler: "text" for the endpoint.