Pink Trees in Unity 5

Hi, I'm having a hard time understanding why my trees are all pink in Unity 5. Both bark et leaves. They were working nicely in Unity 4.5, i tried importing them in the Beta revision 13 and boom, nothing works. I've tried recreating the material, using different shaders, still pink. On one (only one) of the trees the inspector tells me that it don't use a tree shader for it's bark, which is wrong. When I apply its proposition, nothing changes. I also have some trees from packages of the asset store that I bought as examples. None works except those of the Nature Pack 2 from Manufactura K4, if I recompute them, they become pink too. Any idea of the source of this issue ? Thanks in advance.