That happens with Duplicati more or less randomly. I’m just now recreating about 2 terabyte backup, because this error hit me again: “Fatal error => Detected non-empty blocksets with no associated blocks!”.
But that server was running old version of duplicati, so probably and hopefully that issue have been fixed already.
Also the slow database rebuild is strange issue. Sometimes it happens quickly, and sometimes it’s extremely slow. I guess it depends if it’s trying to recover missing blocks or not. Normal restore without local database happens quite quickly, in case there’s nothing wrong with the backup.
But as you can see, the forum is full of similar reports. That’s why I’m always using at least two backup products in parallel.
One case yesterday: