Doc,
This sounds like it's either a cookie issue or a time issue on the user end.
Here at UBBDev a couple months ago, it'd tell me I was logged in, but it wouldn't let me do anything. Heck, it wouldn't even let me see the boards!
I finally cleared all the cookies, and I tried again. I was finally able to get in.
I don't know what it is about the cookies getting corrupted, but they sure do!