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Looking further into this the routing seem to make the form lose track of "PreviousPage".
I would think that using PostBackUrl with EPi is a very rare thing so it's probably something that's not tested.
I usually use non-server forms even when it's a Web Forms project.
When I post a form with Button.PostBackUrl set to an other page than current I get the following error:
In the above case I post from start page to "/other-page". The Alloy site renders the same result.