Translation begins:
"WebDAV Access
If you have a paid tariff connected, you can connect to the service via WebDAV protocol. On your computer, the Cloud will look like a regular file system."
“Duplicati can use WebDAV servers to store backups.”
Have you tried Duplicati’s current WebDAV capability?
How to Fix this Error When Creating a Database Fix Already Created Backup Copy suggests maybe so, although it’s not clear, and the support dialog ended (support through translators adds some difficulty…).
I’m not sure what earlier support topic tried, but what happens if you set Duplicati to use direct WebDAV?
From a resource point of view, it might be faster to get WebDAV going than to create a new storage type, even assuming an API can found. They do have APIs for other things, but the page seems to lack file API.
There is code around the Internet that does files somehow anyway, but in my view official APIs are safer, and Duplicati should consider documenting anything reverse-engineered as having greater risks of failing for possibly long periods (or forever) if the storage provider decides to change something in the protocols.
OneDrive failing : “The remote server returned an error: (410) Gone.” is Microsoft’s announced API cutoff.