The other possibility is that you are allowing your browser to save the username and password and populate it into the login form when you arrive at the forums. Browsers save this information based on the domain name of the site and it is possible that you could run into problems if you also have an account on our main site that is different than the account credentials that you have for the forum. In this situation your browser may load the credentials that it saved for your MyACA account from the main site instead of the account info for the forums. This is something that we would like to address in the future, but do not have a specific timeline at the moment.
I typically use the first method to save my forum credentials and have not had any issues since we have been running the current forum software. Hope this helps to clear up the issues you are experiencing!