"Client error: VDDK error: 10041633552057.NBD_ERR_NETWORK_CONNECT Exception from server: VDDK error: 10041633552057.NBD_ERR_NETWORK_CONNECT Unable to retrieve next block transmission command. Number of already processed blocks: [988]. Failed to perform data backup"
This is the first time I see this message, someone has an idea of the problem.
If that's the only VM that has an issue, try switching a backup mode or migrate this VM to another datastore to make sure there is no issue with the production storage.