from what i have read in the past it is recommended to update the latest available version (in your case 9.1.8) first before going to the next major version.
the upgrade automatically runs with loglevel debug, can you spot an error in the owncloud.log? did you change the number of days activities are kept? It defaults to a year, which accumulates quickly. You can try deleting entries from the activity table older than 30 days manually (after a backup) and then do the upgrade again.