UNITY shows texture image size as 0x0 in preview

My problem is after working normally with importing texture file (png, jpg,…), suddenly after inserting new texture by dragging and dropping, Unity shows no image in preview and its size is 0x0. Strangely, from that time every one of my newly imported images has error. Also, the console window keeps displaying:

  1. Texture is not accessible.
  2. TextureFormat is invalid!
  3. imageIndex < 0 || imageIndex > m_ImageCount
  4. No Texture memory available to upload
  5. Unable to match textureformat to simple format 0 UnityEditor.AssetImporterInspector:Awake()

Has anyone encountered this problem before? Any solution?

I have the same problem currently with Unity 4.5.4
It is only happening in one specific project, others are fine.
Few days ago everything worked, now I get this error for every texture. reimporting all, reinstalling unity, nothing helped.
anyone any idea what to do?