Missing changelogs of packages

[frankenbrunnen]

Hello dear Debian maintainers,

when opening changelog for inconming or previous release changes, it's
not possible to get any result for many packages, e. g. for commons like
firefox or evince:

"failed to fetch the changelog for firefox-esr
URI was:
http://packages.debian.org/changelogs/pool/main/f/firefox-esr/firefox-esr_68.5.0esr-1~deb9u1/changelog"

Why is that and how can it be fixed?

Regards,
Anke