I have the same error here, I can install plex server, but the service won't start because of "unavailable resources".
I guess there is a conflict with a prior installation of plex, but it seems I can't really remove all plex related stuff before reinstalling it (i tried remove, purge, userdel, rm -r of plex lib etc.)
Try: omv-confdbadm migrate conf 5.0.0
Worked like a charm ! Thank you so much 
(even if I don't know how the script missed that line, but I guess things happen)
Clear your browser cache or hit ctrl-shift-R
I already did that, so now I even tried on a new computer of which I never logged into OMV, and I still have the same error messages 
Hello, thanks for this guide. I managed to go through the update process, and everyhting went OK, but I have few erros now.
As soon as I login to the web interface I have pop-ups showing:
"The property 'options' does not exist in the model 'conf.system.network.interface'."
"The property 'recursive' does not exist in the model 'conf.service.rsync.job'."
"The property 'numproc' does not exist in the model 'conf.service.nfs'."
"The property 'localmaster' does not exist in the model 'conf.service.smb'."
In such, I don't have any Network Interfaces showing in "Network" (I had a bond0 configured in OMV4)
And no new data appears in Performance Statistics
