We are evaluating JIRA. I created a user and assigned only "jira-develpers" group to it. After that, the user cannot login – JIRA tells that login or password are incorrect. I am sure I do not forget the password because when I assign jira-users group to this same user as well, I can login all right.

The problem is well repeatable.

I use the Default Permission Scheme. I do not see "log in" permission in the list of permissions but even if the behavior is this by design, I believe, user must have more information about why the access is denied because it authenticates itself correctly.