You are probably going to have to manually remove it from the /etc/openmediavault/config.xml file.
Make a backup copy of the file, and a backup of the OMV system disk. Skip these steps at your own risk.
As the root user edit the file and navigate to the…
New - 2020.12.29
I recommend following this post now for upgrades - Upgrade Scripts for non-interactive major release upgrades (2->3, 3->4, 4->5)
(Quote from ness1602)
On a test OMV 4.x system with the flashmemory plugin installed, this is all I did to…