I cannot reproduce this bug on a Gnome system. Therefore I strongly suspect that this issue is related to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=814190
It looks like policykit-1 is installed on your system, just as is on anarcat's (described in the above mentioned bug report). Please make sure that polkit is running. Can you try running the installer using gksudo? Can you please report back on what "pkexec whoami" gives you? Cheers, u.
