My client said she couldn't get into the site. I tried with my super admin user/pass and it wouldn't let me in either. I tried both front and backend. So looking in my site manager, I see that owner was not selected. I selected myself "admin", but when it saved it gives the JUser::_load: Unable to load user with id: 62 error.

I am still unable to login to it. What is going on? I checked my user and clearly it works because that is the one I am logged into the master site with.

I remade the site and it works, but I'm wondering what happened? Or what could have happened?

In Joomla 1.5, a potential reason that forbid to login is when you are sharing users between different website and that you are not using the same domain than the first one in the slave site definition.

If you are using Joomla 1.6 or 1.7, I don't know the context that could forbid the access.

Hei, i am having the same issue after installing multisite in joomla 1.7,website www.openleech.com catergories europe, asia, americas, oceania that call k2 and feedgator.

This is as a opublic viewing and not registered, could this be related to feedgator cron plugin?

In Joomla 1.5, the user ID 62 generally correspond to the Super Admin.
In Joomla 1.7, the Super Admin is 42

So if you have a problem to load a user ID 62 in Joomla 1.7, I suppose this could be due to something that you read from a Joomla 1.5.
You can effectivelly investigate all the "communication" that you could have between Joomla 1.7 and 1.5.