Short:
I think, Duplicati 2.3.0.4-stable is currently not able to import backup-jobs, when there is a secondary destination defined in the exported file.
Long:
I used Duplicati for years now and currently I am setting up a new PC (Windows 11 Pro). So I installed Duplicati via winget and added a fresh Backup, to run daily on Mon+Wed+Fri, destinations are JottaCloud and OneDrive (separate user, just for backups)
Now I wanted to copy this job twice. One to run on Tue, Thu, Sat and another job just on Sundays. After destroying some of my backups long time ago, I decided to have this system.
To do so, I exported this job to a file and wanted to re-import it right away. But the import fails with error:
Failed to import backup: Could not create an instance of type Duplicati.Server.Serialization.Interface.ITargetUrlEntry. Type is an interface or abstract class and cannot be instantiated. Path 'Backup.AdditionalTargetURLs[0].ID', line 117, position 13.
It doesn’t matter, if I use an encrypted export file or not.
After removing the "AdditionalTargetURLs": [ ... ] part from the not encrypted file, I was able to import the backup job.
As the multiple destination option is fairly new, I guess, that the import function has not been adapted to also support his feature.