I ran into the same problem. I found with an .asp page I could add the following to disable caching on my login form. I think the header you're missing is "Pragma" = "no-cache"

Edit: You actually have the pragma setting, I should have read through the whole post first.

Code:

---

<% Response.CacheControl = "no-cache" %>

<% Response.AddHeader "Pragma", "no-cache" %> 

<% Response.Expires = -1 %>

---