that error means that Docker service is running but the plugin thinks it is not running, or opposite.
You need to stop docker service (service docker stop in terminal) and disable plugin. And then start plugin with manually entered port 42005.
I have had that error a few times, and this usually solves it
Thanks, that fixed it!
I manually shut down the docker service (logged into terminal, and ran "service docker stop"), disabled the plugin in omv gui, restarted the server for good measure. Then I restarted the docker service, manually entered port 42005 in Docker OMV gui (even though it was already set to 42005), turned on the plugin and everything works fine. Hopefully I won't have to do this every time I restart the server.