I’ve been battling an issue for a few days now and finally stumbled upon a workable solution via PowerShell.
One of my customers has an existing Exchange 2007 deployment. The OWA instance is configured to only use NTLM authentication. ExRCA (http://www.testexchangeconnectivity.com) comes back clean, and I can proxy log in to any mailbox on the server.
When configuring the migration endpoint through the Office 365 Exchange Admin Center, I would receive an error that the migration endpoint could not be discovered, even after entering the credentials, server, and RPC proxy server values manually.
The solution ended up with my old friend PowerShell. You can create migration endpoints through it using the New-MigrationEndpoint cmdlet. The key was forcing the authentication to NTLM.
New-MigrationEndpoint -ExchangeOutlookAnywhere -Name NewEndPointName -ExchangeServer EXCHSERVER.DOMAIN.local -RpcProxyServer OWA.DOMAIN.com -Credentials (get-credential firstname.lastname@example.org) -EmailAddress email@example.com -SkipVerification -Authentication NTLM