Some assets are giving an "invalid serialized file header" error

Hey folks! I'm part of a 5 person team working on a game project and each of us owns a copy of Unity 5 Pro. We're using Perforce integration for source control and have had several assets (like the Tag Manager and prefabs) give us the following error: "Invalid serialized file header." We can't identify what exactly the problem is and unfortunately there isn't much discussion about it online anywhere. Recently, we changed our asset serialization mode from "Force Text" to "Mixed", but we don't know if that's causing the issue.