syborg

I forget to say that the error appears if you turn off the groups feature, sorry :/.

@rrecurse I tried your fix, but a new fatal error appeared for me :

Fatal error: Call to undefined function bp_is_group_admin_screen() in .../wp-content/plugins/buddypress-cover-photo/group-cover.php on line 157

This new fix is more elegant, and worked for me :
https://gist.github.com/r-a-y/f54e7a07b71af1c7bfab6e06433da3d6