You'll see this message if it's been more than eight hours since you last signed in to Yahoo! Mail and still haven't signed out. We display this message and ask that you log in again as a safeguard for Yahoo! Mail users, especially those who use public or shared computers.
You might also want to check your account information. If you've chosen to be prompted for a password more frequently than every eight hours, your session expires after the specified amount of time. To change this, follow these steps:
Note: Opening another browser window and logging in to a second Yahoo! Mail account automatically signs you out of the first. When you return to the first account, clicking any link gives you the "your login session has expired" message.
Finally, if the time is incorrectly set on your computer, browsers will have difficulty managing cookie files and generate error messages more frequently.