The problem came up in this ask.sagemath thread and IIRC also in the sage-notebook ML a couple of months back.
The problem is that people need to use the exact same URL that they used earlier to login to any notebook server. I think quite a few people got bit by this or may get bitten by this, especially given the way browser autocomplete works nowadays.
One solution suggested by blutack in the ask.sagemath thread is to put a warning like: "If you are a returning user, please use the exact same URL for this notebook server as you used to log in previously. For example, do not use www.sagenb.org if you used sagenb.org earlier."
Good idea. The other thing to do (with new servers) is to only have one domain name and always automatically redirect to that one domain name. We can't do this with sagenb.org now, though, since lots of people use both domain names (probably).
Actually, I wonder if there is a way to find out how many authenticate with each domain name.