An additional detail: some of the invocations, while failing to produce PAR2s in the backup destination, did dump some .par2 files in ~/, like m2000-q.par2 m2000-q.vol000+190.par2 m2000-q.vol00+34.par2 m4000-q.par2 m4000-q.vol000+163.par2 m4000-q.vol000+200.par2. (Not sure which, I didn't notice them until just now.)
An additional detail: some of the invocations, while failing to produce PAR2s in the backup destination, did dump some .par2 files in ~/, like m2000-q.par2 m2000-q. vol000+ 190.par2 m2000-q. vol00+34. par2 m4000-q.par2 m4000-q. vol000+ 163.par2 m4000-q. vol000+ 200.par2. (Not sure which, I didn't notice them until just now.)