A lot of phones have very little amount of space dedicated to browser cache, which means it fills up fast, and can block the setting of cookies (which is required for logging in).
Try clearing the cache in your phone’s browser, then try logging in again. You may have to clear the cache regularly… like i said, most phones dont allocate much space, and therefore it fills up fast.
Just to confirm, on HTC Touch Pro (Opera Mobile) browsing and logging in on LB works fine.