Until yesterday, when a user opened the OWA website and logged in, it was sufficient to enter the username and password. Now the system is requiring domain\username. If the "domain" is not specified, it places the dns web address of the server ("mailbox.mydomain.org") in the domain position, i.e. "mailbox.mydomain.org\username", which of course fails to authenticate. Now everyone is calling saying they can't log in, because the cached login name fails.
I have made no changes to this system.