I have just added Piranha CMS to an existing ASP.NET MVC 5 site but am having a strange issue. 

If I am logged into the site and then try to access site.com/manager and enter the piranha login details then the page just refreshes and nothing happens (trying with an incorrect password gives me an incorrect password message but correct login details just refreshes page)

If I try accessing site.com/manager without being logged into the site then it logs me in OK the piranha manager area.

Any ideas why this is? Ideally I'd like the user to only need to login once is this possible?