The file format and extension don't match - Excel 365 when opening xll from shared drive only

Question

Hello,

I originally posted this question on the Office 365 forum but because it refers to an xll that is built in C++ (Visual Studio Professional 2019, Version 16.10.3) I was referred here. The problem is that one of internally built xll cannot be loaded in Excel 365 from a shared drive (but it loads fine from the local disk). The same xll loads fine from both share and local disk in Excel 2013. Even if I strip the xll code to the bare bones of xlAutoOpen/xlAutoClose that do nothing, the problem persists. Already tried solutions suggested on the other thread (https://answers.microsoft.com/en-us/msoffice/forum/all/the-file-format-and-extension-dont-match-excel-16/5547d4e6-3e5d-42ab-8a47-604b3bfbfb6a) e.g. to upgrade Office to the latest build, tinker with the TrustCenter settings, etc. but to no avail.

Any help would be greatly appreciated. At this point I'm inclined to think this is an Excel bug, but the fact that I didn't find any other online references to this exact behaviour makes me a bit more skeptical on this assumption.

Thanks and regards,
Claudiu