It’s me again ! 
for the “transmission bug”, it seems that it’s linked to a translation problem in “fr_FR.utf8”
systems (like mine), there is some bug reports here and there.
It seems to happen only with some specifics torrent file (as IPFire’s ISO torrent one), the error message in transmission is:
*** invalid %N$ use detected ***
Abandon
[“Abandon” == “Leaving” so, it’s not a segmentation fault as I first thought]
One solution is to re-compile transmission without “nls” flag, or – more simple – just load (in a term, of course) transmission with ‘LANG=C transmission-gtk’ (or ‘transmission-qt’, of course, depending which version you use (… even if I don’t know if it happens when transmission is compiled against QT library)); with the ‘C’ locale, it’s OK.
I explain all that because I have a day-use of transmission and never met this bug, it is only related with the content of some torrent files of which IPFire’s one is part of, so for french guys in the same situation this is a sort of workaround.
And for IPFire people (devs) maybe it exists a way to generate torrent files that work on french systems using transmission (I get thousand of, here … it’s the first time a torrent file do this).
@IPFire people: Maybe there’s a better place for my post / thread, do what you think is best with it.
(maybe renaming (at least this one) with sort of “Transmission bug with IPFire ISO image torrent file on FR systems” ?)