Your openmediavault-keyring package is outdated. Run omv-upgrade to install the latest packages. This should have been updated automatically with openmediavault 6.4.2.
I think you need to go this way now:
Code# cd /tmp # wget https://packages.openmediavault.org/public/pool/main/o/openmediavault-keyring/openmediavault-keyring_1.0.2-2_all.deb # dpkg -i openmediavault-keyring # apt-get update
Thank you!!!