Just for grins...
Assumming you are using a Windows product.
Run "Control Panel" -- Start-> Control Panel
In Control Panel find and run the "Internet Options" applet.
In the "Internet Options" window, click on the "Privacy" tab.
Then click on the "Advanced" button.
This will open a 2nd window titled "Advanced Privacy Settings"
I recommend that the top checkbox "Override automatic cookie handling" be 'checked'
Then under that select "Accept" under "First-party Cookies" and "Block" under "Third-party Cookies"
Then "Always allow session cookies" should be 'checked'.
Then click "OK" to close that window and "OK" to close the "Internet Options" window.
See if that helps!
Uh, you should probably exit the MLS site and wait an hour or so to let MLS members create some new postings before you come back to check.