Assertion failed: Invalid mask passed to GetVertexDeclaration()

This error came up after reimporting all my files.

I did that because I was getting case errors with my .mat and .mat.meta files. (Unity would automatically capitalize my materials, which probably had something to do with it)

This is the full error: Something with AssetPreviewUpdater???

Assertion failed: Invalid mask passed to GetVertexDeclaration()
UnityEditor.AssetPreviewUpdater:CreatePreviewForAsset(Object, Object, String)

i suspect i will have to reinstall Unity or something, but dont tell me this has happened in MY project