Steps to reproduce:

1. Open attached project
2. Delete the 'torus.prefab' asset
3. Connect the editor to a cache server (just launch the needed version of cache server and type 'localhost' in Unity Preferences/Cache Server)
4. Reimport 'torus.fbx' model
5. Observe the Editor's console, you can see that the prefab was reimported without any problems
6. Check the Project view, to see that the 'torus.prefab' was created and is placed along with the 'torus.fbx', in Assets folder
7. Switch active platform from Win/Mac/Linux to iOS (or any other platform)
8. Observe the Editor's console showing messages:

> reimporting torus.fbx
> CacheServer Asset validation failed 'Assets/torus.prefab'.
> File does not exist library/metadata/f4/f4a496ddd536b4eb694b5c609ab1a93c.tmp_cache

Note: Any additional platform switch will result in CacheServer's error.

Does not reproduce with 5.3.4p1