UNITY 5 lighting issue square shadow

Hello Unity Dev !

When I test, my shadow is really weird. Its a square shadow and If I get closer I can see the shadow of my plane ( see the gif for more info).
One solution I found is increase the shadow distance but I am sure there are a better way because it will cost performance…

Far plane: 7000
shadow distance ( 60 as of now)
shadow near plan ( 2)
Only one directionnal + some area light

Thanks for your help

https://giphy.com/gifs/bug-unity-shadow-light-3og0ILHBIDhwtIxUeA

Anybody to help me ? thanks a lot