I am having the same problem. In my case, it was working yesterday (on a different install), but I didn't install the demo data on my first install, so I deleted my database (nothing in it anyway) and reinstalled. After my re-install, I clicked to go to Admin and encountered this problem. (At first, I thought I had the wrong username or password, but when I changed these, I got the correct error message.) So on this installation, this problem popped up on my first attempt to log in as admin.

I'm wondering if it's because I clicked on secure connection for admin in setup. (I don't think I did when I installed the first time a couple days ago.)

PHP version: 5.2.6
ZenCart version: 1.3.8
host: pair.com
Is your /cache folder writable? yes