Always check the source what supports when trying to mirror! We have lost some time before discovering that our source repository does not support udeb and source packages! If you create a mirror with “-with-sources=true -with-udebs=true” the update process will require files, which may not exists in the source repository if it does not offer udeb or source files and you’ll end up with broken mirror and error for missing file!
Downloading & parsing package files... Downloading http://aptly.example.com/ubuntu/dists/xenial-myrepos/main/binary-amd64/Packages.bz2... ERROR: unable to update: no candidates for http://aptly-master.example.com/ubuntu/dists/xenial-myrepo/main/debian-installer/binary-amd64/Packages found
If you get error for “debian-installer/binary-amd64/Packages” not found, check the source repository if it offers udeb and/or source packages – probably not, so drop your mirror and recreate it including one or the two options