When talking Docker, paths are often confusing (e.g. is it inside or outside the container), but that isn’t what’s shown in the error messages you quoted next. Is Duplicati in a container as well? What image?
Installation with Docker from earlier today has some notes and links to the general Docker information. Specifics of this system may matter, and for checkmk itself, experts are at checkmk.com and its forum:
might be better answered there. Google saw quite a bit of backup, Docker, and container commentary.
Are you able to stop it? Backups of things that stay running risk files disappearing under them, maybe
System.IO.FileNotFoundException: Could not find file "/host/mnt/volume1/data/docker/volumes/check-mk-raw/omd/sites/cmk/var/pnp4nagios/spool/perfdata.1702335593"
(and you can see the difference between that and /data/cmk/monitoring backup, so what’s where?)
With complicated systems with lots of moving parts, the plan is sometimes to take a snapshot (so it sits still while being backed up), and there are careful directions for how to restore (don’t forget that part…).
Facilities for this are provided by the application. Databases are often this way. A live backup may grab something, but it won’t be internally consistent, so might not restore and run. See checkmk information.
When asking clarifying questions, you might not want to specifically say Duplicati, but Docker might be relevant. You need advice for a running (or does that matter?) checkmk for file oriented backup/restore.