We use aptly for mirroring your repository, but since October it doesn't work.
Update script ends with message:
ERROR: unable to update: component 10gen not available in repo [mongo]: http://downloads-distro.mongodb.org/repo/debian-sysvinit/ dist

We think, that the cause is wrong directive settings in this file: http://downloads-distro.mongodb.org/repo/debian-sysvinit/dists/dist/Release
There is
Components: mongodb
and should be
Components: 10gen

Target of your actual settings is URL: http://downloads-distro.mongodb.org/repo/debian-sysvinit/dists/dist/mongodb which doesn't exist.

There are two solutions

• change Components directive from mongodb to 10gen
• rename directory from 10gen to mongodb - http://downloads-distro.mongodb.org/repo/debian-sysvinit/dists/dist and new Debian repo will be deb http://downloads-distro.mongodb.org/repo/debian-sysvinit dist 10gen