Parece que é um bug conhecido do Seamonkey .
Um dos comentários no relatório de bug sugere corrigir manualmente as prefs em about:config
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I think the problem is the leftover valid setting for the news server. Usually you can see the pref for a short time when you create an account manually. (...)
When you cancel the subscription (...) you end up with a mail.server.serverX.valid boolean pref set to false. This seems to confuse the heck out of the account wizard. The simple solution seems just to delete the pref. The not so simple solution would be to make sure that every mail.server.serverX has such a valid pref regardless of it's setting. This seems to work too.
De minha parte, consegui resolver o problema excluindo as duas contas de notícias. Isso fez com que o New Account Wizard restaurasse seu comportamento normal, então finalmente consegui criar uma segunda conta de e-mail.