HTTPS problem after upgrade to 1.4

[SantiBailors]

I just upgraded my installation from 1.3.3.0 to 1.4.0 and followed the upgrade guide. Now I experience this problem: when I type the usual URL to go to my PrestaShop admin board, like

http://www.mydomain.com/myshop/myadmin/

I am redirected to

https://www.mydomain.com/myshop/myadmin/login.php

(for some reason the http became https) and there I get a 404 Not Found:

The requested URL /myshop/myadmin/login.php was not found on this server

, although the login.php file actually exists in the /myshop/myadmin/ folder. So I think that the error is due to the http that becomes https, although I don't know why this happens.

Any suggestion is appreciated.