Not sure if this is the same problem I am having, but, I too had a problem logging in to the admin and as a returning customer to the cart while testing.
One thing that screwed this up was because I originally installed the cart to a domain with just
http://domainhere.com then, to accommodate the SSL Certificate I had to change the default url to
http://www.domainhere.com - and this resulted in breaking the login to everything as the configure.php file was set to just the http:// address.
I was able to fix the admin login by changing the configure file, however, I can't find the customer login file?
If this is not the same problem you are having, sorry for the mixed post!