On my websites I changed my admin username in WP by creating a second admin with the new name, deleted the old one and assigned everything to the new me. Since then none of the gravatars are working on existing posts. E-mail addresses are correct, but WP does not produce a hash code with the image. The image link is missing the hash code and is simply http://www.gravatar.com/avatar/s=32
Things are ok when I write new posts.
I think this is a WP issue...?