It's probably not the controls but rather the support libraries. I have the necessary dll's baked into our installer, and have not had any problems using the mscomct2 calendar, and any machines including Win 10. I'm not using the same VCX (baked my own to suit our purposes), but your problem seems to be with the control, not the class wrapping it.

>I have 2 apps that use the _OLECalendar class of the VFP FFC class _DateTime.vcx.
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>Both work fine on Win7.
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>On Win10, when invoking the calendar, one app works fine. The other crashes with an error: "DdeReq failed C 4006".
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>The two apps use slightly different versions of _DateTime.vcx. However, I rebuilt the problem app with the VCX from the "good" one and the problem persists.
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>In the _OLECalendar class, its .OLEClass property shows as MsComCtl2.MonthView.2 . I'm guessing a "DdeReq" error may refer to Dynamic Data Exchange (DDE) which AFAIK VFP doesn't use directly, but an ActiveX/OCX might.
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>The next step will be to dive into the implementation details of that class in these two different apps, but I'd appreciate it if anyone has seen an error like this before and can offer some ideas.