I GOOGled "Registration needs returnurl DotNetNuke"
The fix is: http://www.datasprings.com/Products/ProductForums/tabid/727/view/topic/postid/1594/Default.aspx
EXCERPT:
The fix is to modify admin/skins/user.ascx.vb, line 142
FROM THIS:
Response.Redirect(NavigateURL(PortalSettings.UserTabId,"returnurl=" & ReturnUrl), True)
TO THIS:
Response.Redirect(NavigateURL(PortalSettings.UserTabId, "", "returnurl=" & ReturnUrl), True)
MY ADDITION:
Similarly, you will have to change the code in admin\security\signin.ascx.vb, on line 629
Response.Redirect(NavigateURL(PortalSettings.UserTabId, "", "returnurl=" & HttpUtility.UrlEncode(HttpContext.Current.Request.QueryString("returnurl"))), True)
because if you go to the login link, then click on register button, the same error occurs if you don't change this line.
Hope that helps.