Hi @ Ryan Wilderman ,
The Mailbox Replication Service was unable to connect to the remote server using the credentials provided. Please check the credentials and try again. The call to 'https://6fbbfcd9-9983-4bc7-8c9f-d44e461f6d24.resource.mailboxmigration.his.msappproxy.net/EWS/mrsproxy.svc' failed. Error details: The HTTP request is unauthorized with client authentication scheme 'Negotiate'. The authentication header received from the server was 'Basic realm="6fbbfcd9-9983-4bc7-8c9f-d44e461f6d24.resource.mailboxmigration.his.msappproxy.net",Negotiate,NTLM'. --> The remote server returned an error: (401) Unauthorized.. --> The HTTP request is unauthorized with client authentication scheme 'Negotiate'. The authentication header received from the server was 'Basic realm="6fbbfcd9-9983-4bc7-8c9f-d44e461f6d24.
resource.mailboxmigration.his.msappproxy.net",Negotiate,NTLM'.
Based on this error message, the issue may be due to the migration endpoint password in wrong on On-Premises end.
I recommend that you refer to the following link to reset the password for the account used to set up the migration endpoint to see if the issue persists:
Mailbox Replication Service was unable to connect to the remote server - Azure365Pro.com
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