I originally talked about this problem here:
The problem is related to Pro Sites. One client is a premium user (= not a free user). There was a problem about the monthly payment so his site's status became "expired pro site".
His subsite is using Domain Mapping and I set this a plugin as a Premium plugin (= for pro users only, not free users).
As a result, he couldn't connect to his back office and the result was the same when he tried with the subsite url (subsite.creersonsite.net) or the mapped domain url (mappeddomain.com). For the mapped domain I think that makes sense, but I don't understand why he couldn't access to the back office of subsite.creersonsite.net
Front-office remained OK though, on both subsite.creersonsite.net and mappeddomain.com
But I guess it shouldn't work on mappeddomain.com, so it's a problem too.