Sorry, let me try again. There are too many moving parts to this, so it’s easy to get different ones confused.
Question was vague. I spoke about downgrading Duplicati, but downgrading would not do OneDrive v2, and continuation on OneDrive (original version) is not possible because Microsoft retired the API, They’d require Duplicati 2.0.4.5 to do backup, but you can continue with the old destination data, and the old configuration (assuming we can get it, for example by temporary downgrade in order to get the config, not do the backup).
Basically you need to move forward on new Duplicati with old config, which ideally would have just happened. We’re apparently still trying to determine whether problem was at upgrade, or if backups had stopped earlier.
That would have avoided current chase, but configuration is clearly not supposed to disappear on upgrades. Losing it through machine loss is certainly possible. Restore can be done if you kept other records, but some details (for example source file selection) may be large enough to not be in, for example, a password keeper.
Restoring files if your Duplicati installation is lost shows how one might deal with a disaster, but it needs some information to get going. If all you have is your OneDrive authentication information, you might also need the Duplicati passphrase. It’s not stored with the encrypted files because that could make encryption pointless…
For any backup, one should test at least a sample restore, and even better is a direct restore to see if you’ve got the right information to do that in an actual disaster. An export of the backup configuration is supposed to also work (saving you some manual typing), but there might currently be a problem with extra option dashes:
Restore from Backup configuration
No and no, but neither is as easy as with some backup providers that have custom-built features to do them.
Backups are typically meant to be used from one computer. You don’t ever want two backing up to the same destination. It’s possible to grab files from a backup when at another computer by use of the same restores mentioned above, but it’s a bit risky because you get closer to the two-computers-on-one-destination failure.
Adopting a backup is most quickly done by moving files, including databases, though backup databases can be (sometimes very slowly) recreated from destination files, assuming you have the necessary configuration. That would be similar to the direct restore’s needs. It builds a database too but it’s only a “partial temporary”.
Migration to new PC - faq wanted is a useful request, had a planned solution, and we haven’t heard back yet.
In my view, the main Duplicati page at https://www.duplicati.com/ shows some of the key concepts behind why things are not as easy as they might be. The “Many backends” means one can’t count on advanced backend features that pre-packaged backup systems attached to specific storage may use, e.g. for backup adoptions. “Strong encryption” is taken seriously, causing passphrase to be stored separately from the files it encrypted. There might be ease-of-use aids later (Duplicati is young), but flexibility and security will probably remain key.
What exactly did you do? If you think you found a 56KB file that used to be Duplicati-server.sqlite, you’d move any current one aside, copy backup 20190116063617.sqlite to Duplicati-server.sqlite, start Duplicati, and see if it sees any backups. If you started it without a downgrade first, it should upgrade the database version, and wind up with the same configuration. Sometimes backups and other things show up slowly in the web UI, so a look at something else such as About → System info to see if anything shows up as unknown may be useful.
You can take a guess at that by seeing what’s present on direct restore unless the config was super-precise. You can also go down the --unencrypted-database path, and find it in the Source table in an SQLite browser.
Duplicati.GUI.TrayIcon.exe explains the options. If you have the default automatic-start-at-logon you have no special options. You can either copy the shortcut and add --unencrypted-database temporarily, or Quit the Duplicati tray icon, then re-launch it manually from a Command Prompt with --unencrypted-database added.
Sounds odd and worrisome. Ordinarily you’d get updated dblock, dindex, and dlist files on every backup run. Did anyone have a look at job logs on that system, or reporting (e.g. emails) to see if backups were running?