Login error

For some reason I can no longer login using (domain)/wp-admin. When I enter that address and gets redirected to the login page, then after submitting my user-name and password, it would get timed-out giving me an error message saying “Firefox has detected that the server is redirecting the request for this address in a way that will never complete.”

I have discovered that, however, if I type in (domain)/wp-admin/ (notice the extra back-slash at the end), then everything works like it should be.

Anyone has any idea why is this happening and how can I fix this? Thanks!