Question: When using the Multiple Site Manager (MSM), when attempting to log in, I enter my username and password and am returned back to the login page. If I log in from one of my other MSM sites, I can log in. What causes this?
Answer: This problem is caused by inaccurate paths somewhere in your CP General settings. Once you set them back to your original domain, you’ll be able to login successfully.
Category:Troubleshooting Category:Multiple Site Manager
If you are having trouble logging into your second(+) sites CP you might need to add your site’s url to the $site_url variable in the path.php as mentioned here.