I just had this same problem but I already use my email address to log on. I ended up signing in to the client area and then headed to the forums afterwards in order to sign in as my username didn't work either.
I have an interesting little problem with seeing the shoutbox on the global scale. If I set the admin and moderator groups to use the shoutbox it only appears on its own page, yet is viewable by all other groups. If I set it so admins and mods cannot use it then the global box reappears.
Some background: until recently it was used only for admins and mods via the tab only, and was hidden to everyone else. I have just re-opened it for all registered members and that's when I noticed this problem. Is there a setting in the database or something that isn't automatically resetting itself? I've uninstalled and reinstalled a couple of times, ensuring the shoutbox database tables were deleted before reinstalling for the second time. I have also checked it with all skins, including the default one, and the same thing happens.