Hi,
ownCloud version downgrades are not supported.
If you still have a backup of your old version, you should be able to restore that.
When doing an upgrade especially a more complicated one it’s always good to work on copy of your production setup to figure out the way. Once you have a confirmed working upgrade path you can apply it to your production setup.
So as I see it right now, you’ve upgraded your PHP from 5.x to 7.3, upgraded your Debian from 9 to 10 and upgraded your ownCloud from 9.1 to 10.3.2
If I was going to do this I would do it like so:
- Create a new Debian 10 VM
- Install old PHP and new PHP on the new VM
- Copy ownCloud installation to new VM with old PHP enabled
- Upgrade ownCloud
- Enable new PHP
- Make sure everything works
- Rerun steps 3-4 but migrate this time
There is a multitude of options available, this is just one example of how to do it. There is no right or wrong really as long as you have a clean, secure, working setup at the end.
Another option could be to upgrade PHP & ownCloud on the old server, so you have cleaner setup on the new server.
Make sure you have your log level on 0, try logging in and check /var/log/apache2/error.log and /path/to/your/owncloud/data/owncloud.log. There must be more than just “login failed”.