GUI accounts are maintained in the database.
Check that the account isn't locked up from the CLI:
# dbc 'select id, username, login_attempts, is_enabled from users;'
and then if any user is disabled, you can quickly do:
for all user accounts:
# dbc 'update users set is_enabled=1;'
for just a single account:
# dbc 'update users set is_enabled=1 where id=###;'