Buddypress Avatars not displaying in default BP theme

Hi folks, weirdass bug it seems, my group avatars in BP are not displaying, even in the BP default theme.

They appear to upload/crop successfully, and the image file is up there in /uploads/group-avatars.

The html for the non-displaying avatars is [ img src="" class="avatar" alt="Practice" ] ie. no source. Any clues?

Many thanks,

Richard