I know. I just had become lazy enough to take the daily notification's subject line ("backup4l has run successfully") as evidence that everything was OK. If I had looked inside the bloody mail I'd have notices that the backup's size was 0B all the time because my self-rolled XZ-compression script failed to add data to the archives it "successfully" created. That's what I meant with "re-testing" - I should better have written "re-validate". My unforgiveable fault was not to look directly at the generated archives after changing the compression from bz2 to xz. Which was pretty pointless anyway as it turned out.