500 Error for admin-ajax.php when editing some images

I’m having the most awkward issue. When trying to update the thumbnail of some images on my site, clicking the “Edit Image” button throws an internal 500 error when loading the admin-ajax.php. I tried this from the post image editor and from the media library area. but this dosnt happen to all the images. some images are ok to edit, and some arent.
and when i go to one of my child blogs and try the same thing, it works fine 100% of the time. its just in my parent blog that i get this error, and not for all my images. sucks that i need to crop the thumbnail for the images where this fails.
anyone have any ideas to why this selective error?