1.5.5 version breaks all plugins permitting the recrop of Featured Image thumbnails - like http://wordpress.org/plugins/crop-thumbnails/ or http://wordpress.org/plugins/post-thumbnail-editor/
When, after cleaning/optimising the database with WP-Optimize, I try to use one of these plugins, they cannot create a new thumbnail relative to the Featured Image like they do normally. If I try on a Featured Image before cleaning database, it works. So I suppose your plugin deleted some important data relative to attachments.
I hope it won't break the use of Regenerate Thumbnails plugin because if so, it's just a BIG problem for my sites if I change my theme !!