OK, first of all, I must disclose that while I’m more tech-savvy than the average computer user, I’m not tech-savvy by the standards of this forum. My knowledge is more practical than technical; I just want things to work so I can focus on my work and, time allowing, on my life interests.
My Duplicati setup is a bit complicated due to lack of space on my computer (a 2015 notebook with a 256 Gb SSD). My Duplicati database is on the SSD, but the backup is on an encrypted folder in the cloud, and the cache for this backup (for the upload, to be precise) is on an external hard drive. This hard drive disconnected yesterday when I made the mistake of plugging one too many devices to the same USB hub. So what must have happened is that the file Duplicati was creating got corrupted.
But if the corruption of one file – among the >5,200 files (127 Gb) making up this accumulated backup – invalidates all the incremental backups (>70) and make further backups impossible, that’s a serious flaw. I tried repairing again. I tried to delete and repair the database. All to no avail.