Seems to work again (thanks to this Github comment for mentioning).
$ for distro in CentOS_7 Debian_8.0 Debian_9.0 Fedora_26 Fedora_27 Fedora_28 RHEL_7 SLE_12 SLE_12_SP1 SLE_12_SP2 SLE_15 Ubuntu_16.04 Ubuntu_17.10 Ubuntu_18.04 Univention_4.0 openSUSE_Leap_15.0 openSUSE_Leap_42.3 openSUSE_Tumbleweed owncloud xUbuntu_16.04 xUbuntu_17.10 xUbuntu_18.04; do echo $distro; wget -q https://download.owncloud.org/download/repositories/production/$distro/repodata/repomd.xml.key -O - | gpg - | grep "^pub" || wget -q https://download.owncloud.org/download/repositories/production/$distro/Release.key -O - | gpg - | grep "^pub"; done 2>/dev/null
CentOS_7
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Debian_8.0
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Debian_9.0
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Fedora_26
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Fedora_27
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Fedora_28
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
RHEL_7
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
SLE_12
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
SLE_12_SP1
pub rsa2048 2013-08-26 [SC] [expired: 2018-08-25]
SLE_12_SP2
pub rsa2048 2013-08-26 [SC] [expired: 2018-08-25]
SLE_15
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Ubuntu_16.04
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Ubuntu_17.10
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Ubuntu_18.04
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
Univention_4.0
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
openSUSE_Leap_15.0
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
openSUSE_Leap_42.3
pub rsa2048 2013-08-26 [SC] [expired: 2018-08-25]
openSUSE_Tumbleweed
pub rsa2048 2013-08-26 [SC] [expired: 2018-08-25]
owncloud
xUbuntu_16.04
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
xUbuntu_17.10
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]
xUbuntu_18.04
pub rsa2048 2013-08-26 [SC] [expires: 2023-08-23]