Acronis Backup Cloud: backup fails with "The network path was not found" or "Cannot connect to the machine where network share ' path' is located"

Backup fails with one of these errors:

Windows error: (0x80070035) The network path was not found

or

Cannot connect to the machine where network share 'path' is located. The machine may be offline or is experiencing communication problems due to a network error or lock-up.