I ran into the same errors as Toppi after following the instruction from Message 38.
I will be upgrading several systems should I add apt-get purge php7.0-common to the instructions just after apt-get purge openmediavault-omvextrasorg
Purging PHP 7.0 after the upgrade breaks OMV5 forcing a reinstall of the OMV plugin and running the omv-salt commands as Toppi suggested. It works just looking for a cleaner way if possible.
I got the same issue, which plugin did you reinstall if I may ask ?