I'm seeing this frequently too. We have data in an sde legacy system and I am trying to transfer updates to aws postgis in the interim. Copies just fail without proper error messages to explain the fault. I have to assume there is a network failure and a geodatabase disconnect, and there is no way to recover without restarting the entire process. Others have told me to export from the db at our end and copy the export to aws S3 attached to the aws RDS geodatabase machine and reimport, but it's a heap of work... and some of these workarounds will work fine in test runs only to become problematic over time.
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