If the option of "Use System Mercurial" is enabled, and you have version 3.7.1 (or possible later version) then trying to, for instance, start a new Release produces the following error:

{{hg flow -y release start Test
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
flow: error: Hg command failed: hg update develop
flow: error: abort: 'dict' object has no attribute 'active'}}

With previous versions of Mercurial it worked fine, and using the "embedded version" works fine too.