I ended up with this error when I executed "sudo apt-get update && apt-get upgrade". I had an old oC with version 6-something before.
I followed the instructions here and felt safe since there should be Migration Test that prevents this to happen.
www-data@hp1:~/owncloud$ ./occ status ownCloud or one of the apps require upgrade - only a limited number of commands are available - installed: true - version: 184.108.40.206 - versionstring: 8.1.9 - edition: www-data@hp1:~/owncloud$ ./occ upgrade ownCloud or one of the apps require upgrade - only a limited number of commands are available Set log level to debug - current level: 'Debug' Exception: Updates between multiple major versions and downgrades are unsupported. Update failed Maintenance mode is kept active Reset log level to 'Debug'
The first time I executed "occ status" I got the error that the www-data user wan't available but that has been resolved by creating a shell for the user.