The backup or restore was aborted - what could it be?
Most likely you have defined a command timeout when you connected to your server. Perhaps without thinking about it. Anyhow, the client (SSMS) will cancel the query after the specified amount of time and return that weird looking error of -2 (which wasn't thrown by the server, it is a client generates message). I tested this by connecting with the "Execution time-out" of 1 as in the picture below and then executed the SQL below.



SELECT 'before'
WAITFOR DELAY '00:00:05'
SELECT 'after'


Zero (0) means indefinitely.