While trying to do a new installation of arcanist, following https://secure.phabricator.com/book/phabricator/article/arcanist/#installing-arcanist, I came upon the above error.
After looking inside the file where the Exception was thrown I saw that it was edited 21 days ago.
So I reverted back to the parent of that commit.
After reverting I was able to follow the other necessary steps again and now Arcanist works as intended.
Not sure if I need to report this here or some other place but just wanted to let someone know that:
- the documentation may be incomplete
- there is some error in the code
- or I did something else wrong that was fixed by reverting.