and why you post here and not in this modthread?
have you leave or uncomment the following modcode in config.php:
//$captcha_chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"; // default
//$captcha_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; // also possible
=> yes => comment it out again
=> no => ok
Kurt