One user can't find a dll at runtime for a ClickOnce-deployed Office app

Question

We've got a VSTO Excel app that we distribute using ClickOnce. We publish it and send the contents of the publish directory to our users, who run setup.exe on their machines to deploy it locally.

This works fine for the vast majority of our users. But one is receiving an exception when he tries to use the app within Excel. Despite successfully running setup.exe and confirming that our dll's were copied to his ClickOnce cache, when he tries to run the app he gets an exception saying that one of our dll's cannot be found:

"Could not load file or assembly 'MyDll.dll' or one of its dependencies. The specified module could not be found."

What is wrong with his ClickOnce deployment?

Answer 1

It turns out the user's virus checker (Symantec) was blocking MyDll.dll from running. He temporarily disabled it, re-installed and ran our VSTO add-in successfully, then re-enabled Symantec and was able to use our add-in normally from then on.