>>>>>Does anyone have any ideas why Excel 2016 and/or Win10 might not like this code?
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>This has proven difficult to debug on machines that display the issue. My gut feel is it's some sort of Excel/Office issue; if I read Tore's last response correctly, he ran my code against Excel 2016 on Windows 10, without error.
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>However, if we can't find and fix the issue on these newer machines/installations we may have to work around it, perhaps avoiding Excel entirely as you suggest.

not really relevant to your question in kept first line, but such issues led us to install a VM for each version of Office Apps we need with access to a "scratch" area to exchange documents - all programmatic handling is considered ok if it does run on one of them.

As you say that the new combo fails on some machines, it is probably some COM/Dll hell, and if the particular machine/VM is reliable, you can always produce the needed files if the desktop currently used can remote the VM.
In practice 95+% is done at personal WS, but if problems crop up, we don't need to fix the specific machine as a tested alternative via RemoteVM exists.