I signed out of Backstage and then tried. I can't get to the actual ticket option for Boston, but I'm betting that they sold out of the Soul tickets. I tried Pittsburgh, and I got to the pull-down menu to select the tickets just fine. Those tickets are not restricted to Backstage members, and there's no password needed. I guess there's a chance that something's gone wrong with that page (in which case an email to support might help), but I think it's more likely that they sold out since there's just one show in Boston this time around.

EDIT - There's no pull-down menu for Boston even when I'm signed onto Backstage either.