While copying from a 6TB network drive to a 8TB network drive, I get Network path not found. When I used WINDOWS EXPLORER to copy from the 6TB network drive to a SSD backup I had no problem copying the data.
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Errors like that come from the operating system, and aren't just something Opus invents. If it happens intermittently, and goes away if you click Retry, then it likely means something to do with the network or the server at the other end is not working correctly, possibly when under sustained load from the file copy.
Searching for "network path not found" finds some articles with suggestions, e.g. Fixing Network Path Not Found Errors in Windows