ClickOnce refusing to launch, claiming the application is unsigned

Background: ClickOnce app that is bundled with the web server of an embedded device. Customers access the web interface, like you would access your router's configuration pages, and hit a button there to launch the ClickOnce app.

Problem: One customer claims he is getting signing error "Cannot continue. The application is improperly formatted. Contact the application publisher for assistance." And in the details, "Your Web browser settings do not allow you to run unsigned applications." This happens before ClickOnce even gets to the part where it downloads the app. Just click the "launch" button and the error is immediately thrown. Customer is running Windows XP and IE8.

The application and deployment manifests are both signed with our VeriSign Class 3 Code Signing certificate. I have check and retested a dozen times with different IE settings, and every time it downloads the ap开发者_开发知识库plication, successfully verifies, and launches. We've had him reflash his device and same thing. No one else has this problem, just him, so I can only conclude that there is nothing wrong with the manifests or their signatures.

His IT department is freakish about security so I strongly suspect that he has some IE settings interfering with ClickOnce, either trying to enforce stricter signing requirements, or blocking it altogether. What could be causing this? Are there some group policies somewhere that are maybe shutting him down?

Try clearing the browser cache.

Go to Internet Options -> Advanced, then click Reset...

For me worked check in Internet Options -> Advanced -> "Allow Software to run or install even if the signature is invalid"