When starting a migration for the first time I had a fail, an error with some database. Maybe it wasn't that but one migration didn't get fully complete so I had to restart. The problem was that now my connection information seemed OK but when starting the migration the Replace showed an old domain from my source site. I notice this and tried to fix it on the custom migration options but the migration always ended up with the old URL. I had to restore the original clear backup on WPengine to be able to get 'connection information' that did not generate me this problem. That seems to be the solution. I hope that this can help you, thanks.