The default timeout on the Apache side is set to 60 seconds and might be too low for some environments.
The timeout value is configurable.
You can edit the following file on the portal:
update the Timeout value from 60 to a higher value, say 300 for example.
Then restart the httpd service
-service httpd restart
Run the migration utility once again and see if the error in question is still getting generated. If so, you can continue to increase the timeout value further and test again.