I experienced the same bug during install. After many repeated attempts I
found this post and tried the install without an internet connection, and it
worked.
May I suggest that this bug would be an important one to fix. Who knows how
many people have been put off Trisquel here at the first hurdle? I would have
had to choose a different distro if I hadn't found this post.
I made a forum account just to post this. Many thanks to the developers of
this excellent free distro, but please fix this bug! (or at least warn people
to disable internet if they get the bug!)