This morning I got an error 80131515 loading my MZ-Tools add-in that I had never seen before, and it was not mentioned in my article HOWTO: Troubleshooting Visual Studio and Office add-ins, so I searched the web.

It happens that after installing the add-in through the setup on a virtual machine on Windows Azure, I downloaded a new build from my web (just the dll, not a new setup), replacing the existing dll. And this error appears when you download a dll in such way, so it is “blocked” by the OS preventing its execution for security reasons. The solution is to right-click the dll file to show its Properties window, and in the General tab, Security zone, click the “Unblock” button.