This almost certainly indicates that you are out of disk space.
After I sign into Big Medium, I'm immediately bumped out of the system after clicking any link in the control panel, with a "session expired" message, even though I have just signed in. What can I do?
This almost certainly indicates that you are out of disk space, preventing Big Medium from saving your login information on the server. Check with your hosting company to obtain more disk space for your web account.