Hi Nicolas,
My problem with org-plus-contrib exhibited itself when using a .emacs.d based on Steve Purcell's, https://github.com/purcell/emacs.d. Specfically, when loading ob-R, I got this error: Invalid function: org-babel-header-args-safe-fn
In response to your email, just now I stripped that config right back to basics, and found that my problem went away, and I was able to load ob-R after all. Sorry, I should have done that before.
Having realised that, I think I should report this against Steve Purcell's .emacs.d, and leave you guys alone.
Thanks for your help.
cheers,
Simon