Hi, I am having this same issue. Can you let me know how you resolved it?
Thanks
OK. I think I resolved this.
It seems to me that the theme being used is not compatible with WordPress 4.0.1
I reverted back to 3.9.3 (not wanting to stay on this), which I am able to log in now.
Just to make sure on this. I updated my installation back to 4.0.1 and it produced the same result of admin redirecting back to log in, so this is why I am thinking the theme is not compatible with new update.
