Migrating from Server 2003 to Server 2019 using Storage Migration Service

I ran this again on a more limited set of 24,205 files, and it failed on about 60.  I grabbed one and looked in depth without finding anything wrong. I can copy the same file pulled from the destination system with just a "copy" statement, so it's not being blocked by anti-virus or disk corruption.

I tracked it down in the event log and the error is simply: 

(64) The specified network name is no longer available.

Bear in mind that every test I have done with other tools, like robocopy or goodsync, have succeeded without error.  If there are network issues they are not showing up in other tools. 

I am aware of this: https://support.microsoft.com/en-us/help/961293/unable-to-access-shares-the-specified-network-name-is-no-longer-availa

But we are running 12.1.1101.401 RU1 MP1, so it's much later.  It is running only on the source, not the destination.  I cannot turn it off due to policy (this is a production site in 24x7 use).  And again, no other tools are hitting that error. 

I am running it again to see if it fails on the same files, but it takes an astoundingly long time.  These folders took about an hour with robocopy (I did not time it precisely), but too Storage Migration about 10 hours.  And I ran SMS in a relatively idle time, and robocopy during prime business hours.

And yes, I am running SMS on Windows 2019 server, fresh install, fully patched, with proxy server, with the SMS component updated to the latest version.

Any ideas?   Is it really this slow?   Is it really this unreliable?