Steps to reproduce:

Site Manager/Sites/New Site Wizard

Step 1: select "new site from template"
Step 2: choose any template
Step 3: enter any data
Step 4: Exception thrown "Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation."

Workaround: instead of working within the page frame, elect to open the frame in a new browser window. Everything works fine then