[Plugin: AppStore Links] Error in function saveImage

After upgrading to 4.4.6 this started to happen.

Error in function saveImage: Could not save the output file '/var/www/vhosts/web.com/httpdocs/wp-content/plugins/appstore/cache/422065992.png' as a png.

Its in old posts, not in new ones.

How can I fix it ?

Thanks.

http://wordpress.org/extend/plugins/appstore/