It looks like we could not connect to the Migration endpoint, this is often resolved by restarting IIS on the Exchange servers ...

It looks like we could not connect to the Migration endpoint, this is often resolved by restarting IIS on the Exchange servers on-premises. Please restart IIS on the Exchange server(s) and then click add again below. 
How to restart IIS: From the Exchange Server(s) click Start, then Run, then type IISReset, and then click OK.