permanent thumbnails error / thumbnails deleted

hi

I’ve been getting a thumbnail memory error on several images on upload, no matter how much I downsize them, it just keeps happening and there’s no thumbnails of the pic anywhere.
Message is: “a failure occurred. exceed memory limit. require: 66,77megabyte”.
Also, upon uploading new images, existimg thumbnails are being deleted. Very annoying.
Another bug that keeps happening is that deleted images keep showing up.
Thanks for any hints.
http://ww.wp.xz.cn/extend/plugins/nextgen-gallery/