Same troubles here. In fact of my 4 units, two of them with the old MAC-557 interface are working fine, the other two (MAC-567 and MAC-587) are cold dead.
I think that the problem is that old units use a different server (leswifidata.meuk.mee.com) than new one (production.receiver.melcloud.com), the latter is KO.
Waiting for Mitsubishi solving the problem…