Magnus,
Did you verify that the User(s) had the Web Portal enabled as I think by default in 6.3 SP7 it is set to No?
Also, you may want to verify that the new credentials match the password policy set under System > Maintenance Accounts or they may not work properly. I think by default it is set to just use a minimum of 6 characters unless you have changed it; which you should.
What exactly is the screen showing?
Thanks,
TE