So I’d just done a nice install of Exchange 2013 into an Exchange 2010 Enviroment.
Everything is runing great and all users have been migratd to the new server.
So now I have to migrate the publicfolders.
Technet has a great post on the subject here
But I kept recieving this error “Couldn’t connect to the source mailbox”
The Solution you ask?
It turned out that after migrating all users I deleted the Users databse from the source server and this caused the error.
So all you need to do is create an empty user database on the source server and then continue the public folder migration.
Hope this helps you out as that error message was not easy to diagnose.