Login error: Oops... There was an error on the server.

I have a private (all galleries are invitation only) smugmug site with ancestral family photos. I've shared it with about 20 people so far, and several of them are having the same login problem. They get the invitation, create their guest smugmug account without issue, then when they try to log into my site they get the error on the login screen when trying to submit their credentials, "Oops... There was an error on the server" and are unable to login.

I've seen this error reported elsewhere and found nothing concrete other than recommendations of clearing one's cache and cookies. Which prompted a theory: I wonder if this issue is related to the fact that my site has a custom domain name. Could it be that when the user creates their guest account, they're on the smugmug domain and perhaps some authentication cookie gets set with that domain name. Then, when they come to my site with the custom domain and try to log in, there's some sort of conflict as the authentication is confused between my domain and the smugmug domain?

Regardless, this is not a good user experience - most of the people involved here have no idea what "cookies" or "cache" are, so to make the normal workflow require they clear all that is a major stumbling block. Anything I can do or smugmug can do to make this a more seamless experience?

Thank you!