An "Invalid Cookie" error message will sometimes be displayed if the cookie on your system does not match the Yahoo! ID that you are using with Messenger. To solve this problem:
- Exit the error message.
- Point your browser to http://my.yahoo.com
- Sign in using your Yahoo! ID and password. This will set a valid cookie on your system.
- In the Messenger window, click Login an choose Login/ Change User. Verify that you have use the same Yahoo! ID and password with both the Messenger and My Yahoo!.
- Click on the Login button to return to using the Messenger.
This should solve the problem. If it does not, make sure that your computer displays the correct date and time (as the cookie is sensitive to this information).