Good point. So I set up yet another account for a different member of my party, and once again, that did not work. I can see that my 2014 ressie is populating into the new accounts; for example, on the new account, it "remembered" that my son has a birthday celebration, which I had entered in my previous account. So, switching people doesn't work. It still populates the error, as the error is tied to me ressie, not to the person who sets up the account.
To expound, however; what I've got to lose is that I am running out of email addresses which I can register. I tried registering my husband, and found that his two email addresses are already in the system, presumably from when I was having trouble last year. So that makes at least six accounts registered between the two of us, just to secure one dining reservation per year for two consecutive years. I hope Disney intends to fix this glitch; it's super annoying.