Yeah, looks like it's the cross domain request. In production, this will not be a problem as everything is in the same place. But for development it is distinct systems. Boy this is a PITA (not Ext JS - the whole security thing).
If I was you, I'd go with Ext.Direct.Mvc for this. It lets you avoid all of the security/configuration hassles of dealing with WCF. Basically with Ext.Direct, you'll have an ASP.NET MVC app instead of a WCF project. You can create multiple controllers, and those controllers have actions/methods. So you'll create methods to do the usual CRUD stuff. It's much more lightweight than WCF, so you'll spend less time dealing with WCF and more time focusing on what your app needs to do. With an ExtJS app, the bulk of your logic will be on the client anyway, so all your server methods will probably be doing anyway is the usual CRUD, validation, and then returning back some Json.
Where Direct comes in is, it will generate method stubs for you to call these methods easily. It also handles the conversion of parameter input into the appropriate .NET types on the server side, etc. So for each MVC controller method, you'll have a method stub created for you in your client side code in the format Server.ControllerName.MethodName - for example, Server.Customers.Load(). It's pretty easy, and it also gives you a nice perf boost because Ext.Direct bundles multiple server calls that happen in quick succession into a single packet, which helps if you're network latency isn't very good.
I think the author has moved the code several times over the last few years as development has gone on so all of those are probably different versions of the same thing. There is a huge thread on it in the Ext.Direct forms. Reading the thread, or at least the more recent stuff at the end of it, will point you to whatever the newest is. Here is the link:
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