by Tim Hunt.
that sort of error is always harder to diagnose.
It is most likely a time-out caused by one part of the process taking an unexpectedly long time. (Most web browsers give up after 5 minutes of the server appearing not to do doing anything, even if the servers is still busily processing away without sending back any data.)
Diagnosing that will involve looking at the particular data in that backup file, and how Moodle processes it.