First of all, login for this forum and for the subtitle site is NOT the same. You should use the username and password you use to login on
www.opensubtitles.org
Obviously, double check if you spelled your username and password correctly.
Further, I assume you are using some API software, but I don't know which one. Here are some tutorials how to login using various software:
viewtopic.php?f=11&t=17153