Single user won't stay logged in
On an older site (probably 3 years old), the client notified me of a peculiar. We have one, and so far as we know only one, user that when the login, they are taken to the correct 'favorites' page, showing all the info one would expect. However, the moment they click away from that page, it's as if they have logged out. They cannot add to cart or checkout, as it immediately kicks them to the login page as if they are not logged in.
I have tested with other accounts with a similar access level, and they function fine. It's only this one account that seems to have the issue. I've looked at their user info in the user table, and I don't see anything out of the ordinary.
Any thoughts on things to check? I'm at a loss for how to troubleshoot.