As you have probably read in a previous article, the EndPoints that you setup for Exchange Online to use are vital for a successful migration. Here I am providing a quick walkthrough tutorial on how you would go about troubleshooting those if they do present themselves with an error during migration.
Noting that you may not know there is a problem with an EndPoint until you kick off a migration. This offers a very quick way to test them before you start.
Follow this video and you can check out the EndPoint locations in the console, perform a PowerShell check that they work, and/or create a new one if needed.
Feel free to drop me a note anytime with questions and comments, I am always happy to receive feedback and help out.
Lastly, here is the link to the other article I mentioned, with some more in depth details about setting up the EndPoint with PowerShell.