Right, it looks like the issue is PHPBB was case sensitive passwords, but this new forum is not, it forces inputs to lowercase. This means that if you had an uppercase character in your password, you would generate an incorrect hash and it would fail your login.
Hopefully tonight I can edit one line of code and fix it all in one go. This will now mean your passwords ARE case sensitive. The reason your uppercase generated passwords worked is because they were a trusted generated string that wasn't passed through any filters.