HCW Error when configuring the mail flow

I also work with Kevin Eyer and wanted to provide an update:

We are being told that since we already manually added a connector to our Tenant to allow mail flow from Office 365 cloud only mailboxes to our mailboxes on-premise which is required to continue mail flow from O365 Tenant users to our on premise users when we added our domain, that is causing the error.

Upon further reveiw of the logs by MS, they show that the root cause of the issue is that there is already an existing trust relationship with your onPrem environment. This can be seen in the log, if you look for this error: " PowerShell failed to invoke 'Set-FederatedOrganizationIdentifier': A trust relationship has already been defined for this organization." in additon to the original error posted by the OP.

In a nutshell, they are claiming that the connector we created a few days ago (which was required because as soon as we added our domain to an already established Tenant {due to a merger} it broke mail flow from O365 to our user on-premise and a connector had to be created to establish mail flow again) is causing the error even though I didn't realize adding a connector creates a Federation Trust? ..... Insert sarcasm. However, in the end the fix they are claiming is to remove our connector, wait 24 hours for it to "purge" the connector from the Tenant then rerun the HCW. Of course this will break mail flow from O365 to our on-premise during this time.  We did remove the connector from our Tenant and reran the HCW but still errored. They are claiming it may take up to 24 hours to remove and do not support to manually purge the connector via PS commands...sigh

Curious to hear others response on if you are setting up Hybrid to a brand new Tenant with no domains or is your Tenant already established and you have added the domain to the Tenant along with the connector required then runnign the HCW.

This is definitely holding up our migration and discussing internally on the next steps or using a 3rd party tool.