It looks like the "educated guesses" do not help in this case. Something 
breaks without leaving an evident trace at the level of backuppc and of 
Linux - you need more evidence (I agree, after having eliminated the 
most likely explanations for such a diffusely manifested error).

The next step I would attempt (and which has already been suggested) is 
to try and get more direct feedback from rsync: look at the command-line 
that backuppc generates and than run this command manually from a shell 
- maybe adding a -v to get more output. There may be some hope that 
rsync produces a helpful message which is not caught by backuppc and by 
the OS and which might give you a hint on what is going on.

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