Buddypress and "are you sure you want to do this?" screen

Helle there, I get a screen with ”are you sure you want to do this?” when I click ”accept” or ”reject” to a group membership.

I have been reading tons of forums and posts, and I know it is something related to the nonce control.

But how to fix it???? I have a clean brand new installation of latest versions of both wordpress and buddypress, default theme of BP… so what could be wrong?!

Thanks for any help you may give…
Francesco