Considerable effort was put into troubleshooting this issue on affected machines, without success.
The final resolution was to replace Excel automation with the use of Greg Green's VFPx class:
https://github.com/ggreen86/XLXS-Workbook-Class .
My gut feel is the future will see increasing restrictions/lockdowns on inter-process communications, function calls, automation etc both by Windows and by security/AV programs. My recommendation would be to eliminate Excel automation where feasible and replace it with this class.
Regards. Al
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