Hi there,
I have a user who has the following popup every time they open Excel, even if they are not opening a document directly:

The version of Office is Microsoft 365 MSO 16.0.14131.20296 64-bit.
I have found a number of older posts related to this issue (such as this one) that all recommend creating a reg key under HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Common\Security, however as I am on the latest Office, I only have 16.0. I have tried to create the "Security" key and then the DWORD value in my corresponding 16.0 folder but it makes no difference.
I have also tried both a quick and online repair of Office.
Any other suggestions would be greatly welcome.