phew, solved it. The problem was indeed that for some reason my key wasn’t added anymore to my ssh-agent (see my previous reply).
running these commands added the key to my ssh-agent:
# start the ssh-agent in the background
eval "$(ssh-agent -s)"
# Agent pid 6273
ssh-add ~/.ssh/id_rsa
now Capistrano is able to deploy my Rails app again.
Just curious now what caused my ssh-agent to be without identities. All I know, is that last week I executed some OS X updates, maybe that caused the problem. Not sure though.
greeting and thanks to Jared for putting me on the right track to fix this.
Anthony