Checking your other computer was a good idea. For the affected one, I wonder if the ransomware got it somehow, while maintaining original date? If you have the affected computer drive, you could see if that pattern happened there, meaning file date is reasonable but file content is encrypted by the ransomware.
Restore your OneDrive shows that if you have Office 365 you might still be able to undo remote damage.
Confirming the file is very messed up is probably best done with a hex viewer such as Frhed. If you drag some .zip.aes file onto its window, a properly built AES file should have some readable text, for example
It’d be interesting to look at one of the ransomware-bitten files to see if it leaves any signature info inside. Without that, we’re still not sure if ransomware hit your backup or if somehow Duplicati put up a bad file, however once again having a second computer can be useful for comparison, and apparently that’s OK.
EDIT:
made me hope the computer itself had no file access to OneDrive. If it had access, ransomware did too. You could look on your other computer to see if Duplicati’s backup files can be seen in your File Explorer.