We recently upgraded Domain Mapping (and other plugins) and today I upgraded Domain Mapping again to version 18.104.22.168, but even at the next lower version some sites began displaying a url like the one shown below during login and hanging at this url. I do not recall seeing the "dm-sso-endpoint" before.
Regardless of the mapped domain we are trying to log into this is the url that the login process hangs at.
If the login process does complete then attempting to access the dashboard area from the top menu bar on some sites redirects the user to the network login page even though they were already logged in when they accessed the dashboard link on the menu.
We have the multidomains plugin version 22.214.171.124 active as well.
I also noticed that several sites with Jetpack connected to http://www.wordpress.com suddenly reported a strange error that something was wrong with the site connection to Wordpress.com even though nothing seemed wrong. Deleting the existing connection and reestablishing it seems to resolve that error message even though it appears to reconnect the same as before.
I have tried different combinations of Domain Mapping settings in the Network Dashboard without resolving the issue.