yesterday we noticed that some devices running Chaos Calmer 15.05 weren't able to download certain dependencies after boot since it seems that all files/directories in http://downloads.openwrt.org/snapshots/trunk were moved/deleted and the server is answering an HTTP 404 error.
Do anyone knows if this is a temporal issue; and in that case, when is it estimated to become available again? Or where can we find a mirror of the missing repository?
That URL was never a repository to a release. SNAPSHOTS/TRUNK refers to a development build. Any packages found there were obsoleted when the kernel was bumped in that trunk. You will have to install a released version to have a permanent package base.