https://twitter.com/jottacloud shows a problem on May 18. Does time fit your backup time? Also shown at https://status.jottacloud.com/ and https://status.jottacloud.com/history.
It depends on how many files are unavailable, and until you do a thorough check, e.g. with a larger or all sample count on The TEST command with a URL from Export As Command-line, nobody knows extent. Note, however, that downloading all files to find out if they actually are there will be a pretty slow operation.
You can live with a small number of lost dblock by deleting them officially (using Jotta UI, however if it can rename them, then changing the prefix to something other than duplicati can hide them), then running The LIST-BROKEN-FILES command in Commandline (after some edits to convert from backup syntax) will tell you what backups of what source files those files affect. After that, The PURGE-BROKEN-FILES command will remove the broken source files from their backup versions, and you’re normally done then.
This all requires a working Jottacloud though, and we don’t know if we have one until some tests get done.
Recovering by purging files describes all this, but it’s not something you want to do unless forced. Safer to explore the issue as suggested, IMO, and ideally get it with a Jottacloud app and go talk with Jotta support.
I’d note that your 404 files seem to be dindex not dblock, but it’s an incredibly small sample. If the only files with 404 errors are dindex, and if your DB is original (no attempt to Recreate it), then deleting or hiding the dindex files via Jottacloud web UI, and then running Repair should upload new dindex (if Jottacloud works).
I don’t know if this is one of them. If Duplicati were trying to download a file that wasn’t there, I’d say maybe it lost track of its files, but you said that the files that 404 are there, and download from the Jottacloud web?
Again, I can’t guide you without any information on how many files have gone unavailable. I can’t tell if they will return someday courtesy of Jottacloud. I can’t promise you that starting the fresh backup will work, but a small one would be an easy test on basic sanity of things. The CommandLine tools would also be tests.
We need to figure out where things stand. Maybe Jotta changed their API or something, although I’d think there’d be more reports of it, even from Duplicati uses, however https://usage-reporter.duplicati.com/ has Jottacloud use at off-the-charts small (unless I missed it – you can go look), SSH (SFTP) has the largest usage of any clearly remote protocol. You could certainly try Duplicati with Lima Labs to see how it works.
2.0.6.1 does have a rare regression with servers where it winds up using ecdsa keys. A fix is in progress.