Apt update && apt upgrade fails - 404 for Ubuntu_16.04/./all/owncloud_10.0.0-1.1_all.deb

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Steps to reproduce
1.apt update
2.apt upgrade

Expected behaviour
I'd expect the packages to be found, downloaded, and applied.

Actual behaviour
Instead I get

root@cloud-ourspace-io:~# cat cloud /etc/apt/sources.list.d/owncloud.list
cat: cloud: No such file or directory
deb https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04/ /
root@cloud-ourspace-io:~# apt update
Hit:1 http://archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://security.ubuntu.com/ubuntu xenial-security InRelease
Hit:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:4 http://archive.ubuntu.com/ubuntu xenial-backports InRelease
Ign:5 https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04 InRelease
Hit:6 https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04 Release
Reading package lists... Done
Building dependency tree
Reading state information... Done
34 packages can be upgraded. Run 'apt list --upgradable' to see them.
root@cloud-ourspace-io:~# apt full-upgrade -y
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following package was automatically installed and is no longer required:
Use 'apt autoremove' to remove it.
The following packages will be upgraded:
apt apt-transport-https apt-utils cloud-init dnsmasq-base dpkg libapt-inst2.0 libapt-pkg5.0 libgtk-3-0 libgtk-3-bin libgtk-3-common libpam-systemd libpci3 libsane libsane-common
libsystemd0 libudev1 makedev owncloud owncloud-deps-php7.0 owncloud-files pciutils python3-software-properties python3-update-manager snap-confine snapd
software-properties-common systemd systemd-sysv ubuntu-core-launcher udev update-manager-core update-notifier-common wget
34 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 27.8 MB/53.3 MB of archives.
After this operation, 43.3 MB of additional disk space will be used.
Err:1 https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04 owncloud 10.0.0-1.1
404 Not Found
Err:2 https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04 owncloud-files 10.0.0-1.1
404 Not Found
Err:3 https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04 owncloud-deps-php7.0 10.0.0-1.1
404 Not Found
E: Failed to fetch https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04/./all/owncloud_10.0.0-1.1_all.deb 404 Not Found

E: Failed to fetch https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04/./all/owncloud-files_10.0.0-1.1_all.deb 404 Not Found

E: Failed to fetch https://download.owncloud.org/download/repositories/stable/Ubuntu_16.04/./all/owncloud-deps-php7.0_10.0.0-1.1_all.deb 404 Not Found

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

Server configuration
Operating system: Ubuntu
Web server: apache
Database: mysql
PHP version: 7.0
ownCloud version (see ownCloud admin page): owncloud/now 9.1.4-2.1 all [installed,upgradable to: 10.0.0-1.1]
Updated from an older ownCloud or fresh install: current version is fresh install

If I presume that https://download.owncloud.org has mirrors behind it, then clearly one (or more) is not up-to-date and needs to fixed.

Please report such issues always directly to the repository maintainer reachable via https://github.com/owncloud/core/issues

In this case your issue might match the one already reported here: