no gravatars on multisite install

Hi
I have a WP multisite and I am missing the gravatars. I maintain multiple WordPress websites and everywhere my gravatar is showing on users.php (users without gravatar have the default one) but on 1 multisite install gravatars are not showing, not the default one not set ones. Like gravatars are switched off. The theme is (like on many other sites I maintain) a Genesis childtheme – normally gravatars are supported. But on none of the sites of this multisite installation of at network – users the gravatar is showing. CHecked everything. I know at least my gravatar should work, but since also the default ones are not showing, there most be something wrong. Hope anyone can tell me where to look.