hm can you confirm that the credentials you're using are actually existing in the database? or do you have a default admin set by Security::setDefaultAdmin()?
is this member linked to the ADMIN-permission?
another debugging method would be to clear all session- and domain-cookies for the specific site (we encountered problems before where redirect-urls were kept in cache which prevented restricted users from accessing the "right" cms-areas)
OK problem solved. Luckily for some reason the database had my email address - where it got it from i do not know - apart from i have a form that sends to it whilst i was testing.
So that meant i could reset the password (altho i never had one for the email address) and hey presto - were back in!
So from now on i am going to make sure that instead of using the default admin and password during development - I will create a new administrator with an actual email address that I can access and change the password