Ok, it's a bit clearer now, I'm no apache expert! :^)
I'll look into that, though it seems to me that there is no AuthUserFile and AuthGroupFile on our system.
What's really troubling is that on 2 out of 3 of theses machines, everything works ok, so why does Apache accept authentication for 2 of them but not the 3rd? Should the AuthUserFile and AuthGroupFile be on this 3rd machine, or only on the server where backuppc is installed?
Thanks for your help anyway!