At the very end of an upgrade from version 5 to version 6 of OMV, I received the error
Updating workbench configuration files ... Restarting engine daemon ... Errors were encountered while processing: grub-pc W: --force-yes is deprecated, use one of the options starting with --allow instead. E: Sub-process /usr/bin/dpkg returned an error code (1)
To finish the install, the solution is to simply run this command:
sudo dpkg --configure -a