Solved.
Similar to the issue I had here: Admin interface assets forbidden by nginx
This was a permissions issue during installation. For some reason the permissions on the roundcube config file that the setup process creates were incorrect. This meant that roundcube was defaulting to invalid configuration.
note the permissions on config.inc.php below:
root@box:~# tree -p /usr/local/lib/roundcubemail/ -L 2
[drwxrwxr-x] /usr/local/lib/roundcubemail/
├── [drwxrwxr-x] config
│ ├── [-rw-r-----] config.inc.php
│ ├── [-rw-rw-r--] config.inc.php.sample
│ ├── [-rw-rw-r--] defaults.inc.php
│ └── [-rw-rw-r--] mimetypes.php
├── [-rw-rw-r--] index.php
The fix was:
chmod 755 /usr/local/lib/roundcubemail/config/config.inc.php