I migrated my config files from OH2.5.x to OH3.x by copying over the folders and files from my ‘old’ v2.5.x Pi3 openHabian machine to a new v3.x Pi4 openHabian machine.
However I seem to have got the owner / permissions mixed up on those folders, sub-folders and files. Can someone kindly advise me how to reset the owner-group, owner, and r/w/x permissions on the new machine?
have you tried openhabian-config -> fix permissions ?
Thanks for the tip; I hadn’t tried that, so I tried it just now, but unfortunately it gives the following error message…
There was an error or interruption during the execution of "10 | Apply Improvements" Please try again.
^
But I think that error was something to do with the permissions for the MQTT broker only. 
In the meantime I tried this as well…
sudo chmod -R 774 /etc/openhab