The file format and extension of 'addin' don't match error

[AE]

I have a strange use case where addin will always display the following error about mismatch of the file format

This normally happens when you have Excel 64bit and you try to load 32bit version of addin or vice versa. But in this case both versions of addin do not load and always display this error. Excel is 64bit. 

Exactly the same build will load normally on other systems with 32 or 64 bit version of Excel.

What could possibly cause this behaviour? 

Thanks!

[Govert van Drimmelen]

Maybe blocked by an anti-virus?

-Govert

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