I had this issue today. I recently logged in to one of my old accounts on a website which linked to a fictional family tree I created. When I clicked on the link, that message appeared. However, when I logged in, I was able to access the tree fine. I think it is caused by backend maintenance on the site's part, or a lot of traffic coming in. (Alternatively it could be caused by privacy settings, but this was not the case for me.)
If it re-appears for you then you shouldn't worry too much, either way it's not your fault but the management of the systems. Just be weary if it appears again.
