After deleting an rsync backup task in order to change user permissions and then creating a task with the same destination path I get the above error.

Solution:

1. delete the synobkpinfo.db file
2. rename the backup folder e.g. backup to backup_old
3. create a new task with the same destination backup
4. copy new synobkpinfo.db into backup_old
5. delete backup
6. rename backup_old to backup

I also investigated the .db file using SQLite Browser:

#synology_to_truenas_backup

https://community.synology.com/enu/forum/1/post/124168