Default logging is often too-brief.one-line summaries, dropping wanted details which are on next lines. This really hurts ability to debug one-time problems. Reproducible ones just require additional logging.
Viewing the Duplicati Server Logs gives one of the broad ideas, but there are many different logs kept.
If error is reproducible, could you please see if About → Show log → Live → Retry can show details? Alternatively, for unattended logging, you could instead set Advanced options –log-file and –log-file-log-level=retry. Yours is in a different spot, not the after-backup verification. It looks like compact is trying to pull down something old that has turned into wasted space. If you can get a name, can you also look at its date? Unfortunately the compact gets dblock files which contain actual data from the source files, so permanent damage is hard to recover from. If dblock was created before about Sep 2019, throttling bug was there. If this is too complex, you can perhaps look over your destination files by date to see how far back they go, and whether damage from throttling might have been done. I can’t say, without evidence.
Asking people to post their backup files is risky. Even if normally encrypted, buggy situation might differ. The hexdump command is more a Linux thing, but Windows has some nice GUI editors such as Frhed.
What Storage Type did you set on your Destination screen? I trust some (and some destinations) more. There could also be some unfixed bug that nobody has been able to track down enough to lead to a fix.
If you like, start over, but if “the rate of backups that end up damaged is very high”, what others do you have? “I also have encountered the error under discussion. This thread it’s the first result of my google search.” isn’t clear on whether you keep hitting this one (but just now looked), or if it’s a variety of them.
Thanks for that. Canary is the early warning system, so I hope you aren’t hitting anything newly buggy. Although it may vary unpredictably, recent Canary has been doing great for me in terms of backing up.