Can’t set default thumbnail

I really like this plugin. I do notice a few bugs today: I can pick an image from the Media Library as my default thumbnail for posts that don’t have images, but the settings page returns “Image not exists. Please check your image from media library. You can add new one or push ‘Save Changes’ button to get default image directly.” I have tried this with multiple images in my Media Library.
I’m also unable to set a custom thumbnail size of 90×90.

Thanks!