You should think about moving to OMV 5.x since 4.x is EOL'd.
I assume it will fix the error as well than?
If I would like to update, I could follow your steps.
Do I need flashmemory to be installed? I don't have that yet and I have no clue how I can install it.
sudo apt-get --yes install omv-extras-flashmemory does not seem to work. It cannot find the packet/plugin.
omv-extras should be installed by: sudo wget http://omv-extras.org/openmediavault-omvextrasorg_latest_all4.deb
Followed by: sudo dpkg -i openmediavault-omvextrasorg_latest_all4.deb
And at last: sudo apt-get update