Guacamole currently assumes that the root directory of the SFTP server is "/", and that this directory will be readable. In practice, this is not guaranteed to be the case. In addition to variation in file permissions, SFTP servers may not actually have a directory at "/". Though the standard requires that absolute paths begin with a "/" (and that the path separator is "/"), it does not require that "/" be a real directory nor that it be readable.
In the case of a platform which lacks a root directory entirely (such as Windows or certain routers), this means that you end up with legitimate paths like "/C:/some/directory/" even though "/" does not exist and thus cannot be listed.
For the sake of supporting such platforms, and for the case where SFTP access should be restricted to a specific directory and its subdirectories, the root directory should be configurable.