Incorrect. I tried joining several months ago and it gave me this error regardless of the 5 different emails I used. I put it down to the time difference, and that during midnight to say 4AM the system kept giving me that error. Doing it at a different time the next day worked fine.

They should replace that error msg with something to do with systems maintenence.

Interestingly I found that at the same time AA was telling me they had no flights on a certain day, so it must be common in the industry.