What you can do on your own to try to track it down is go through the registration pages until you're at the final page and fill in the fields... after which it will run a name availability check on the name entered. Every time you try a different name, it should check again... so if you have any idea of the name, you'll be able to see if it's registered without attempting a login.
Note though, that if you still have your old install, your account name will be stored in a local file under gamedata. Specifically, public.store and private.store contain an account name as the first (tab-delimited) field of each line.
You should only get that message if your game is unable to contact the webserver for the information needed (account download will also fail, probably the reason you figured your account name was forgotten). Check your firewall/antivirus settings and process affinity to ensure it isn't being blocked.
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Note though, that if you still have your old install, your account name will be stored in a local file under gamedata. Specifically, public.store and private.store contain an account name as the first (tab-delimited) field of each line.