I have the exact same problem as Trollineto. I've tried your hint, but no success.
I've gone thru your backuppc 4 times scupolously, with my colleagues controlling me to be sure I didn't make any mistakes or typos, and I end up always with the same issue. Strangely when i did it the first time it worked perfectly, then all of a sudden backuppc stopped working because it can't establish the ssh connection anymore. So i've tried to rebuild the ssh tunnel again, following your instructions on point 5 of your how-to, but no luck. It's like the backuppc server not being able to find the authorized key entry on the client.
Is there any log I can check to find out the problem?