Details

When multi site manager is installed with EE 1.6.8 you are unable to login to the control panel for specific MSM sites.

Steps to reproduce:
1. Install EE 1.6.8 with multi site manager
2. Setup a second site in MSM
3. Setup a member group for the second site.  Be sure to allow control panel access for this member group.
4. Create a new member and assign them to the group created in the previous step
5. Before logging out of the control panel, go to the ‘Sites’ tab and select the main site (not the secondary site).
6. Logout of the control panel
7. Try and login using the new member account that should only have access to the secondary site

RESULT:
You get the error message on the login page: “You are not authorized to perform this action”

Additional Info
I’ve found that when you switch sites in the control panel, a cookie is set to store which site you are currently on (the cookie name is exp_cp_last_site_id).  This seems to be controlling which site you will see when you login.  If you first login as a super user, switch to the secondary site then logout, you would be able to login to the secondary site using the new member account that should only have access to the secondary site.