I'm not able to reproduce "Return code 0" error. Did you receive this error message every time you try to select a Master Page? Maybe you can describe some steps to reproduce or send me a video.
You can also give a try on our demo - http://demo.sitefinity.com/Home.aspx
According to your problem with changing existing Template to a different Master Page I logged it as a bug with id 106589. I can suggest you a workaround. When you are creating a template select "Don't use template (start from scratch)", then use Actions > Base template and select your master page. After that you will be able to change this master page with another.
I think you should not have any problems renaming a template, can you provide more information about this errors you receive, any screenshots or examples would be nice. Thank you in advance!
All the best,
the Telerik team
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