Display Moreroot@OMV:~# grep -ir backport /etc/apt/*
/etc/apt/apt.conf.d/01autoremove: "linux-backports-modules-.*";
/etc/apt/apt.conf.d/01autoremove-kernels: "^linux-backports-modules-.*-4\.14\.0-0\.bpo\.3-amd64$";
/etc/apt/apt.conf.d/01autoremove-kernels: "^linux-backports-modules-.*-4\.15\.0-0\.bpo\.2-amd64$";
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
/etc/apt/preferences.d/omv-extras-org:Pin: release a=stretch-backports
Strange. You don't have a file to enable the backports repo. Try:
omv-mkconf apt
then post the output again:
grep -ir backport /etc/apt/*