As many others I'm guessing, I accidentally upgraded to version 3.0.15 from 3.0.13. This resulted into a non working omv installation and the uninstallation of a lot of extras plugins. I've successfully reverted back to 3.0.13. Because I think it can be helpfull for others I'll explain how to do this.
1. SSH into your OpenMediaVault machine as root
2. Run the following commands one by one:
apt-get install php-json-schema
dpkg -i ./openmediavault_3.0.13_all.deb
service openmediavault restart
3. Login into OpenMediaVault via the web interface. If you've had OMV-Extra's plugins installed they won't be listed under services anymore.
4. Reinstall your plugins via the 'Plugins' tab, or install them from terminal using:
apt-get install openmediavault-yourplugin1 openmediavault-yourplugin2
5. The configuration of the plugins will still be there but you might have to disable/enable the plugins to get them fully functional again. I had to do this with openmediavault-mysql to get the web-management interface working again.
That's all. No need to panic :).
Hope this helps!