Hello Compostela,
Is there an interest to know when update failed? I mean, RC2 is 3 months old today and rolling is probably much better.
Starting from an old ISO is possible but how do you want to reproduce the successive updates? Some packages have been updated several times; as soon as you'll update one of them you'll download the last update. Believe me, the best way is probably to install the last rolling.
Concerning me, I try to re-install systematically new ISOs after reformat the partition; it takes a few time but thus I can test ISO in frugal and full installation. I never keep rolling more than 2 or 3 updates.
Recently, I installed the 2014/08/04 rolling ISO; it still works perfectly after several updates, yet rolling ISOs from 2014/08/17 doesn't work properly on my computer, even after updates, but rolling ISO from 2014/80/18 works fine... So, it seems that after udate(s), SliTaz is not really like it'd be after an installation with a new ISO.