I am facing an issue with SSO/Cross-domain login not working. I am using both Domain Mapping & Multi-Domains plugin.
If a super admin who is logged in to the WP MU at the main domain (say Domain A) tried to visit the dashboard of his another website which is hosted on subdomain of a different domain (Subdomain of Domain B) and is mapped to its own custom domain (Domain C), he is either redirected to login page or "Incorrect or out of date login key" error.
Main Domain - Domain A
Super Admin logs in at Main Domain (Domain A)
He clicks on My Sites > Site X > Dashboard
(Here Site X is hosted on a subdomain of Domain B and is mapped to its own private domain, Domain C)
He is redirected AGAIN to the login page or to the Incorrect Date error. The URL of the login page looks like:
And no matter how many times the admin fills in the username and password, he is always redirected back to that page.
Though sometimes clearing cookies and cache before the relogin temporarily solves the issue (and sometimes it does not)
Here is a link to a thread I created in 2014 for the same issue (for reference solutions):