I have tried upgrading from Owncloud 10.3.2 to Nextcloud and no matter the Nextcloud version I used, it failed.
I read the issues with same topics and it did not help, because of incompatibility versions, I lost hours and almost lost my Owncloud server.
Considering that you don’t have many users accounts, for my case my users where connected from Zimbra server so I did not need to have their logins:
So that’s what I did:
I setup a new Nextcloud v.16 server
I backed up my Owncloud files to an external HD
I connected my external HD to the new NC server
I updated successfully the Nextcloud server to v.17 to be sure this works
I configured it to have an external storage for the HD
I opened the files and could browse all the backed up ones from my HD
I installed the Impersonate app to be able to login to the users directories
I copied to each user’s folders the files from the backup ones.
So when a user goes on the Open Drive in Zimbra to connect to the file server, he could see all his files again.
Maybe it is a longer workaround but at least safe and reliable. No more bad surprises.