The _bzr_ plug echoes "added: somefile.mdwn" when it adds somefile.mdwn to the repository. As a result, the redirect performed after a new article is created fails because the _bzr_ output comes before the HTTP headers.
The fix is simply to call `bzr` with the _--quiet_ switch. Something like this applied to _bzr.pm_ works for me:
46c46
< my @cmdline = ("bzr", $config{srcdir}, "update");
---
> my @cmdline = ("bzr", "update", "--quiet", $config{srcdir});
74c74
< my @cmdline = ("bzr", "commit", "-m", $message, "--author", $user,