This started to happen to me when I added custom fields to the directory listing from your directory plugin. Every time I update a post it displays "This field is required." for the featured image. Even if the featured image is filled in.
I am able to save the post once the image removed and readded.
The version of custompress installed is the one that came with the Directory plugin. I've since installed the custompress plugin manually too and am still having the same issue.