Duplicati Developer documentation says
The backends encapsulate the actual communication with a remote host with a simple abstraction, namely that the backend can perform 4 operations: GET, PUT, LIST, DELETE. All operations performed by Duplicati relies on only these operations, enabling almost any storage to be implemented as a destination.
Duplicati has its own filename design, and expects the backend to be able to access files of such names.
The pixeldrain API documentation seems to say that uploads returns an Id. Downloads are done by the Id.
While in theory a backup program could save such mapping, the site is meant for file sharing, not backup.
This possibly also explains potential file deletions after 30 days . See For how long will my files be stored?
Beyond that, Duplicati’s idea of LIST is a directory listing. Theirs is giving back a list that you had uploaded.
EDIT: Controlled DELETE also doesn’t specifically exist, so that’s zero out of four required API capabilities.
Maybe other reads will be different, but this site appears to me to be a very unsuitable Duplicati integration.