This is something I absolutely h*** about ExtJS. All the internal changes constantly breaking code...

...so the problem is that I was creating a static model function using the previously proxy instance in a model.

Code:
Ext.define('App.model.app.Server', {
    extend: 'Ext.data.Model',
    proxy: {...}

    statics: {
        connect: function() {
            this.prototype.proxy.doSomething()
        }
    }
});
Now, the proxy is only the config object, but no real instance any longer. I guess this was introduced to use less memory and to create less object instances, or? Is there any other reason for this?