new vps host and bp adminbar login no longer works

I have just recently migrated a couple of sites to a new VPS hosting package.

Since moving them I can no longer access the login page using the adminbar login link - it return a 404 for one site it just automatically redirects to the home page for the other.

I can go directly to the login.php page and login.

After I have logged in I also have the same issue with the logout link on the adminbar.

Has anyone come across this before - it's very frustrating.