Answer
This isn't usually an actual invalid-certificate problem — it's more commonly caused by enhanced security settings in the Office/Project application itself being stricter about ClickOnce-style add-in deployment manifests. Add the IFS server's URL to the Trusted Sites zone in the machine's browser/Internet security settings, and also check the Trust Center settings specifically within MS Project itself, adding the same URL there as well if it isn't already trusted — this combination typically clears the message without needing to reissue or reinstall any certificate.