"The email address or password is incorrect" EVERY time I try to access my inbox

Issues with signing in; each time I click on the shortcut in Microsoft Messenger that takes me to my inbox, I have to manually sign in, despite the fact that the "Keep Me Signed In" box is checked.  After I manually sign in, if I close the window and attempt to access my inbox via the shortcut again, I have to sign in again.  It happens each time, and is very annoying when I need to check my e-mail multiple times a day.  The message on the sign-in page says either my e-mail address or password is incorrect, and to try again, when I know for a fact that both my e-mail and my password are correct.  I found someone with a similar issue, and it was suggested to set the browser not to delete history upon exit, but my browser is not set this way, thus this is not the problem.  Help would be HUGELY appreciated, as I've been putting up with this for months with no idea what to do.