Please try rclone, as mentioned above. Because Yandex supports many protocols, including S3 for which cost is apparently unattractive, I checked for more confirmation they use Yandex disk, and found this URL:
https://cloud-api.yandex.com/v1/disk
Accessing the API for Yandex disk shows cloud-api.yandex.net/v1/, so this supports that they match. The API has been unchanged since 2015 and their Java SDK is the same. API doesn’t look well-cared-for.
I notice rclone forum has a report that it’s slow. I wonder if Yandex is throttling the very-low-priced service, which is not that unusual for other services. Typically such services try to get you to where they want you.
Failing that, the good times may end with a service shutdown such as Amazon Cloud Drive discontinued.
On the other hand, Wikipedia only mentions they created WebDAV issues for some clients. Try another?
Since October 18, 2019, Yandex.Disk has started throttling WebDAV, causing some clients to time out,[4] the official page redirects to the Yandex.Disk 3.0 client page, support writes: “Yandex.Disk is a personal service that is not designed to be used as an infrastructure element. For such tasks, we have Yandex.Cloud"[5]
On the chance that they are nicer to rclone for some reason, rclone also has WebDAV. Maybe it works…
Trying things that already exist is certainly worthwhile, and support is on rclone forum. If that works, then Duplicati may work (with slightly fewer features), and it avoids investing in old API for use they don’t want.
If some new developer wants to contribute code, and preferably support, there are a few nuget packages.
Comparing with the official Yandex Disk SDK #9 is from one. I notice official SDK is last updated in 2013.
I’m not a developer, but one can’t just integrate any old third-party package to Duplicati. Things need to fit.