Office installation won't open files downloaded from a browser

When I open an office file (I've tested Word / Excel) that I downloaded from a browser (chrome, ff, Edge), regardless of the source (gmail, dropbox web download) I get the following error:

This is on a box with:

• Windows 10 (genuine license)
• Office 360, installed locally - not the online version. (also licensed)

If I take the same word file, open it in another computer and then re-save it in a shared dropbox file, I can open it from the problematic computer no problem. It seems that it only happens when the file is downloaded from a browser on the same computer.

Is there some protection mechanism that I'm unaware of?