same issue here with Outlook 2007 with SP - add in will not be loaded and throws an runtime error on loading.
Using Add-In-Spy shows us the error: "die assembly wird von einer laufzeit erstellt die aktueller als die derzeit geladene ...", which means in English "This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded"
It Looks for me like an issue with .Net, but we are using nav 2013 and so we have .net 4.5.1 installed on this Client.
Could it be an issue with interop?