Minimal steps for reproduction:
$ sudo apt install appstream [..] $ sudo apt -qq update E: Failed to fetch http://mirror.gsi.de/distrib/debian/dists/stretch/main/dep11/Components-amd64.yml 404 Not Found E: Some index files failed to download. They have been ignored, or old ones used instead.
This does not really look fatal, ie. it does not mention Packages files.
$ sudo apt dist-upgrade Reading package lists... Done Building dependency tree Reading state information... Done Calculating upgrade... Done 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. $ sudo apt purge appstream [..] $ sudo apt -qq update 18 packages can be upgraded. Run 'apt list --upgradable' to see them.
Kind regards, Christopher